Peaceful engineers seen working outside (6)
I believe the answer is:
serene
'peaceful' is the definition.
(similar in meaning)
'engineers seen working outside' is the wordplay.
'engineers' becomes 're' (abbreviation for Royal Engineers).
'working' is an anagram indicator.
'outside' means one lot of letters goes inside another (some letters go outside others).
'seen' anagrammed gives 'sene'.
're' put inside 'sene' is 'SERENE'.
(Other definitions for serene that I've seen before include "Peaceful and contented" , "Calm, peaceful, tranquil" , "Calm and at peace" , "Peaceful and calm in manner" , "Calm and unclouded" .)