Wizardry of fighting man in rainproof garment (5)
I believe the answer is:
magic
'wizardry' is the definition.
(I've seen this before)
'fighting man in rainproof garment' is the wordplay.
'fighting man' becomes 'gi' (term for US soldier).
'in' means one lot of letters goes inside another.
'rainproof garment' becomes 'mac' (short for macintosh).
'gi' put inside 'mac' is 'MAGIC'.
'of' is the link.
(Other definitions for magic that I've seen before include "Conjuror's art" , "Sprinkling of fairy dust" , "Marvellous" , "Enchanting quality" , "Wizardry - wizard!" .)