Bill plugging Royal Opera House? Something fishy here (5)
I believe the answer is:
roach
'something fishy here' is the definition.
(roach is a kind of fish)
'bill plugging royal opera house?' is the wordplay.
'bill' becomes 'ac' (short for account).
'plugging' is an insertion indicator (I've seen this in other clues).
'royal opera house?' becomes 'ROH' (abbreviation for Royal Opera House).
'ac' inserted into 'roh' is 'ROACH'.
(Other definitions for roach that I've seen before include "member of family carp" , "Fish of the carp family" , "Small freshwater fish related to carp" , "Fish - part of a cannabis cigarette" , "creepy-crawly" .)