Soldiers aboard shipshape English vessel (7)
I believe the answer is:
trireme
'vessel' is the definition.
(trireme is a kind of vessel)
'soldiers aboard shipshape english' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'aboard' means one lot of letters goes inside another.
'shipshape' becomes 'trim' (synonyms).
'english' becomes 'E' (abbreviation as in OED).
're' going within 'trim' is 'trirem'.
'trirem'+'e'='TRIREME'
(Other definitions for trireme that I've seen before include "Ancient Greek or Roman war galley" , "Greek warship" , "Roman/Greek galley" , "Ancient rowed ship" , "Ancient galley with three tiers of oars on each side" .)