Royal Engineer eats new fish (7)
I believe the answer is:
snapper
'fish' is the definition.
(snapper is a kind of fish)
'royal engineer eats new' is the wordplay.
'royal engineer' becomes 'sapper' (I've seen this before).
'eats' means one lot of letters goes inside another.
'new' becomes 'n'.
'sapper' enclosing 'n' is 'SNAPPER'.
(Other definitions for snapper that I've seen before include "Fish; marine turtle" , "Person photographing" , "Fish; photographer" , "(Often red) food fish" , "sharp-toothed fish" .)