Seen about to get engineers calm (6)
I believe the answer is:
serene
'calm' is the definition.
(I know that calm can be written as serene)
'seen about to get engineers' is the wordplay.
'about' indicates anagramming the letters.
'to get' means one lot of letters goes inside another.
'engineers' becomes 're' (Royal Engineers).
'seen' is an anagram of 'sene'.
'sene' placed around 're' is 'SERENE'.
(Other definitions for serene that I've seen before include "Totally unagitated" , "Calm and composed" , "still" , "Calm and unruffled, like a princely title." , "Tranquil, unruffled" .)