Romeo enters school with a poker (4)
I believe the answer is:
prod
'poker' is the definition.
(I've seen this in another clue)
'romeo enters school' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'enters' is an insertion indicator.
'school' becomes 'pod' (term for a group of marine mammals).
'r' placed into 'pod' is 'PROD'.
'with a' is the link.
(Other definitions for prod that I've seen before include "Provide stimulation" , "Stimulus" , "Push with pointed instrument" , "Spur or goad" , "Goad, stimulate into action" .)