Seaman heckles Henry the redcap? (8)
I believe the answer is:
tarboosh
'the redcap?' is the definition.
(red felt hat)
'seaman heckles henry' is the wordplay.
'seaman' becomes 'tar' (Jack Tar is an informal term for a sailor).
'heckles' becomes 'boos' (both are negative responses from a crowd).
'henry' becomes 'h' (symbol for the unit of electrical inductance).
'tar'+'boos'+'h'='TARBOOSH'
(Other definitions for tarboosh that I've seen before include "top gear" , "Fez-like hat" , "Fez-like cap" , "Fez" .)