Seem upset wearing G-string for such a melody (5,4)
I believe the answer is:
theme song
'such a melody' is the definition.
(theme song is a kind of melody)
'seem upset wearing g-string' is the wordplay.
'upset' indicates an anagram.
'wearing' means one lot of letters goes inside another.
'g-string' becomes 'thong' (I've seen this before).
'seem' anagrammed gives 'emes'.
'emes' going within 'thong' is 'THEME SONG'.
'for' is the link.
(Other definitions for theme song that I've seen before include "Characteristic melody often repeated in musical" , "Signature tune" , "Give me Sunshine" .)