Excellent gunners and engineers (4)
I believe the answer is:
rare
'excellent' is the definition.
(I've seen this before)
'gunners and engineers' is the wordplay.
'gunners' becomes 'ra' (Royal Artillery).
'and' says to put letters next to each other.
'engineers' becomes 're' (Royal Engineers**).
'ra'+'re'='RARE'
(Other definitions for rare that I've seen before include "Very lightly cooked, meat say" , "not dense" , "Rear up, being scarce" , "like Ben Jonson, Jack Young thought" , "difficult to find?" .)