Military cap or style of pike (4)
I believe the answer is:
kepi
'military cap' is the definition.
(I've seen this in another clue)
'style of pike' is the wordplay.
'style of' indicates an anagram (I've seen 'style' mean this (style can mean to shape or design)).
'pike' is an anagram of 'KEPI'.
'or' acts as a link.
(Other definitions for kepi that I've seen before include "French army cap" , "Service cap" , "French military hat" , "Flat-topped military cap" , "French military headgear" .)