Placid engineers seen working outside (6)
I believe the answer is:
serene
'placid' is the definition.
(synonyms)
'engineers seen working outside' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'working' is an anagram indicator.
'outside' is an insertion indicator.
'seen' with letters rearranged gives 'sene'.
're' put into 'sene' is 'SERENE'.
(Other definitions for serene that I've seen before include "Fair" , "Peaceful and calm in manner" , "Tranquil, unruffled" , "Calm and relaxed" , "Calm and composed" .)