Gilbert & Sullivan initially aren't terribly funny (7)
I believe the answer is:
strange
'funny' is the definition.
('strange' can be similar in meaning to 'funny')
'gilbert sullivan initially aren't terribly' is the wordplay.
'initially' suggests taking the first letters.
'terribly' is an anagram indicator.
The first letters of 'gilbert sullivan' is 'gs'.
'gs'+'arent'='gsarent'
'gsarent' with letters rearranged gives 'STRANGE'.
(Other definitions for strange that I've seen before include "Odd - rum" , "Slightly odd, a bit weird" , "Queer" , "Weird, eerie" , "Odd or unknown" .)