Unusual interest, initially envious, taken in swimmers? (6)
I believe the answer is:
fetish
'unusual interest' is the definition.
(eg an unusual sexual interest)
'initially envious taken in swimmers?' is the wordplay.
'initially' indicates taking the first letters.
'in' is an insertion indicator.
'swimmers?' becomes 'fish'.
The initial letters of 'envious taken' is 'et'.
'et' put inside 'fish' is 'FETISH'.
(Other definitions for fetish that I've seen before include "An abnormal object of sexual desire" , "Obsession" , "Object worshipped as having a spirit" , "charm" , "Object worshipped by primitive people or of abnormal sexual interest" .)