Foreign ruler hosts small function for composer (7)
I believe the answer is:
rossini
'composer' is the definition.
(I've seen this before)
'foreign ruler hosts small function' is the wordplay.
'foreign ruler' becomes 'roi' ('king' in French).
'hosts' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'function' becomes 'sin' (mathematical function).
's'+'sin'='ssin'
'roi' placed around 'ssin' is 'ROSSINI'.
'for' acts as a link.
(Other definitions for rossini that I've seen before include "Composer born 29 Feb 1792" , "he produced harmony" , "Italian composer, 1792-1868" , "Composer of The Thieving Magpie" , "whose barber was in Seville" .)