Soldiers trapped by mother's ambition (5)
I believe the answer is:
dream
'ambition' is the definition.
('dream' can be a synonym of 'ambition')
'soldiers trapped by mother's' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'trapped by' means one lot of letters goes inside another.
'mother' becomes 'dam' (female parent of an animal).
're' going into 'dam' is 'DREAM'.
(Other definitions for dream that I've seen before include "Part of sleep" , "Aspiration" , "Think vaguely" , "Bedroom vision?" , "Sleeper's vision" .)