Supreme commander involving gunners in good time (5)
I believe the answer is:
craic
'good time' is the definition.
(I've seen this before)
'supreme commander involving gunners' is the wordplay.
'supreme commander' becomes 'CIC' (commander in chief).
'involving' indicates putting letters inside.
'gunners' becomes 'RA' (Royal Artillery).
'cic' going around 'ra' is 'CRAIC'.
'in' is the link.
(Other definitions for craic that I've seen before include "In Irish bars you'll get this" , "Irish pub fun?" , "good time" .)