Flatfish occasionally ready (3)
I believe the answer is:
ray
'flatfish' is the definition.
(I've seen this before)
'occasionally ready' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
The alternating letters of 'ready' are 'RAY'.
(Other definitions for ray that I've seen before include "Broad flat fish; beam" , "Sunbeam or flat fish" , "Beam; man's name" , "Flatfish or sunbeam" , "Beam - skate" .)