Engineers wooden fret (6)
I believe the answer is:
repine
'fret' is the definition.
(I've seen this before)
'engineers wooden' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'wooden' becomes 'pine' (pine is a type of wood).
're'+'pine'='REPINE'
(Other definitions for repine that I've seen before include "Fret, be discontented" , "Express discontent, fret" , "Plain" , "Be in a state of anxiety" , "Fret, about a tree perhaps?" .)