Join outfit protecting knight (4)
I believe the answer is:
knit
'join' is the definition.
(I know this)
'outfit protecting knight' is the wordplay.
'outfit' becomes 'kit' (synonyms).
'protecting' is an insertion indicator (inserted letters are held or protected).
'knight' becomes 'N' (chess abbreviation).
'kit' going around 'n' is 'KNIT'.
(Other definitions for knit that I've seen before include "Use needles and wool" , "Cause to unite" , "Unite; use needles" , "Link closely together" , "Make a sweater, maybe" .)