Persuade helmsman to grab key (4)
I believe the answer is:
coax
'persuade' is the definition.
(to coax someone is to persuade them)
'helmsman to grab key' is the wordplay.
'helmsman' becomes 'cox' (coxswain on a boat).
'to grab' means one lot of letters goes inside another.
'key' becomes 'A' (musical key).
'cox' going around 'a' is 'COAX'.
(Other definitions for coax that I've seen before include "Wheedle, cajole" , "Flatter" , "Carefully manipulate" , "Gently persuade" , "Wheedle, persuade by flattery" .)