Poke rook protected by whale group (4)
I believe the answer is:
prod
'poke' is the definition.
(I know that poke can be written as prod)
'rook protected by whale group' is the wordplay.
'rook' becomes 'R' (chess abbreviation).
'protected by' means one lot of letters goes inside another (inserted letters are held or protected).
'whale group' becomes 'pod' (I've seen this in another clue).
'r' put into 'pod' is 'PROD'.
(Other definitions for prod that I've seen before include "Stimulus" , "Provide stimulation" , "Give a reminder" , "Urge or stimulate" , "Poke or spur" .)