Gilbert's partner seen in foreign villa in blazer (8)
I believe the answer is:
sullivan
'gilbert's partner' is the definition.
(I've seen this before)
'foreign villa in blazer' is the wordplay.
'foreign' indicates an anagram (foreign order of the letters).
'in' is an insertion indicator.
'blazer' becomes 'sun' (I've seen this in other clues).
'villa' is an anagram of 'lliva'.
'lliva' going inside 'sun' is 'SULLIVAN'.
'seen in' is the link.
(Other definitions for sullivan that I've seen before include "Gilbert's partner" , "The Lost Chord composer" , "Sir Arthur" , "Savoy Opera composer" , "Composer of music for Ruddigore, d. 1900" .)