Molly perhaps is in fine hotel (4)
I believe the answer is:
fish
'molly perhaps' is the definition.
(molly is a kind of fish)
'is in fine hotel' is the wordplay.
'in' is an insertion indicator.
'fine' becomes 'f' (abbreviation).
'hotel' becomes 'h' (phonetic alphabet: alpha, bravo, charlie etc.).
'f'+'h'='fh'
'is' inserted into 'fh' is 'FISH'.
(Other definitions for fish that I've seen before include "Ruff" , "Cod, dab, eel and ray" , "occupant of tank" , "Burbot or turbot, for example" , "Aquatic vertebrate" .)