Vision of engineers penetrating barrier (5)
I believe the answer is:
dream
'vision' is the definition.
(dream is a kind of vision)
'engineers penetrating barrier' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'penetrating' indicates putting letters inside.
'barrier' becomes 'dam' (dam is a kind of barrier).
're' put within 'dam' is 'DREAM'.
'of' acts as a link.
(Other definitions for dream that I've seen before include "Somniate" , "Distant ambition" , "Fantasise, imagine" , "Night-time fantasy" , "Trance" .)