Convert for gender realignment, but not at first (8)
I believe the answer is:
exchange
'convert' is the definition.
(I know this)
'gender realignment but not at first' is the wordplay.
'gender realignment' becomes 'sex change'.
'but not at first' means to remove the first letter.
'sexchange' with its initial letter removed is 'EXCHANGE'.
'for' acts as a link.
(Other definitions for exchange that I've seen before include "Short conversation or argument" , "'Swap, trade (8)'" , "Swap, barter" , "Altercation" , "Convert" .)