Join outfit guarding nation's leader (4)
I believe the answer is:
knit
'join' is the definition.
(knitting is a kind of joining)
'outfit guarding nation's leader' is the wordplay.
'outfit' becomes 'kit' (kitting is a kind of outfiting).
'guarding' is an insertion indicator.
'leader' suggests taking the first letters.
The initial letter of 'nation' is 'n'.
'kit' enclosing 'n' is 'KNIT'.
(Other definitions for knit that I've seen before include "Join together (with needles)" , "Put together" , "Make a woolly" , "mesh" , "Unite closely" .)