Patrol boats, say, protected by cannons at each side (7)
I believe the answer is:
cutters
'patrol boats' is the definition.
(I've seen this in another clue)
'say protected by cannons at each side' is the wordplay.
'say' becomes 'utter' (to utter something is to speak or say it).
'protected by' indicates putting letters inside (inserted letters are held or protected).
'at each side' suggests removing the centre.
'cannons' with its centre taken out is 'cs'.
'utter' going into 'cs' is 'CUTTERS'.
(Other definitions for cutters that I've seen before include "Scissors, maybe, or fast sailing-ships" , "Sloops" , "Boats; tailors" .)