Somewhat cross, I niggle composer (7)
I believe the answer is:
rossini
'composer' is the definition.
(I've seen this before)
'somewhat cross i niggle' is the wordplay.
'somewhat' indicates the answer is hidden within the clue.
'ROSSINI' is hidden within 'cross i niggle'.
(Other definitions for rossini that I've seen before include "whose barber was in Seville" , "Largo al factotum composer" , "Composer born 29 Feb 1792" , "Composer of The Thieving Magpie" , "Composer of the opera William Tell, d. 1868" .)