Dream - always one welcomed by engineers (7)
I believe the answer is:
reverie
'dream' is the definition.
(reverie is a kind of dream)
'always one welcomed by engineers' is the wordplay.
'always' becomes 'ever' ('ever' can be a synonym of 'always').
'one' becomes 'i' (Roman numeral).
'welcomed by' indicates putting letters inside (I've seen this in other clues).
'engineers' becomes 're' (Royal Engineers).
'ever'+'i'='everi'
'everi' put inside 're' is 'REVERIE'.
(Other definitions for reverie that I've seen before include "Daydream or fantasy" , "A day dream; a word used in the first ever 2 dn" , "Aimless train of thought" , "Brown study" , "Daydream or preoccupation" .)