Gunners and engineers are hard to find (4)
I believe the answer is:
rare
'hard to find' is the definition.
(I've seen this in another clue)
'gunners and engineers' is the wordplay.
'gunners' becomes 'RA' (Royal Artillery).
'and' is a charade indicator (letters next to each other).
'engineers' becomes 're' (Royal Engineers).
'ra'+'re'='RARE'
'are' is the link.
(Other definitions for rare that I've seen before include "Might steak be" , "Seldom seen or met" , "Unusual(ly fine)" , "of low density" , "Especially good" .)