This qualitative case study analyses the professional identity development of an in-service teacher educator (ISTE) through her conceptualisations and tensions. The participant possesses multiple degrees in different fields (BA in physics teaching, MA and PhD in Educational Administration) and became an ISTE in English Language Teaching (ELT) with this interdisciplinary background. We mainly focused on her lived experiences of developing as an ISTE in ELT in the context of in-service teacher education (INSET) in Türkiye. The thematic content analysis of an in-depth face-to-face interview suggests that resolving tensions early may be needed for ISTEs to persevere in their new job. In the study, this solution was found by establishing legitimacy through academic achievements and finding a source of assurance by showcasing her content knowledge in speaking English in front of the practising teachers. Yet, as the rest of the tensions indicate, not all are meant to be resolved. Some could be productive and may drive ISTEs to invest in themselves and improve their practices. Therefore, tensions could also be seen as an opportunity for professional learning.