Sea-monsters hid inside flowers (7)
I believe the answer is:
orchids
'flowers' is the definition.
(orchid is a kind of flower)
'sea-monsters hid inside' is the wordplay.
'sea monsters' becomes 'orcs' (I've seen this in another clue).
'inside' means one lot of letters goes inside another.
'orcs' enclosing 'hid' is 'ORCHIDS'.
(Other definitions for orchids that I've seen before include "Glamorous tropical flowers" , "Flowers from a rich sod" , "Fine tropical flowers" , "Showy plants" , "Chords I turn into flowers" .)