Composer in group losing heart (4)
I believe the answer is:
bach
'composer' is the definition.
(Bach is an example)
'group losing heart' is the wordplay.
'group' becomes 'batch'.
'losing heart' means to remove the middle letters.
'batch' with its middle removed is 'BACH'.
'in' acts as a link.
(Other definitions for bach that I've seen before include "Johann Sebastian . . . . , great composer (4)" , "German organist and composer, d. 1750" , "Welsh term of endearment" , "JS --, baroque composer" , "Goldberg Variations composer, d. 1750" .)