Gunners and engineers are not common (4)
I believe the answer is:
rare
'not common' is the definition.
('rare' is the opposite to 'common')
'gunners and engineers' is the wordplay.
'gunners' becomes 'RA' (Royal Artillery).
'and' is a charade indicator (letters next to each other).
'engineers' becomes 're' (abbreviation for Royal Engineers).
'ra'+'re'='RARE'
'are' acts as a link.
(Other definitions for rare that I've seen before include "Might steak be" , "not often spotted" , "Infrequent like underdone meat" , "Occurring infrequently" , "Not easily come by like undercooked meat" .)