Engineers get careless and rest (5)
I believe the answer is:
Here is my best explanation:
'rest' is the definition.
(I've seen this before)
'engineers get careless' is the subsidiary indication.
'engineers' becomes 're' (abbreviation for Royal Engineers).
'get' is a charade indicator (letters next to each other).
'careless' becomes 'lax' ('lax' can be a synonym of 'careless').
'and' is the link.
(Other definitions for relax that I've seen before include "Unbend", "Ease", "Chill out", "Put your feet up", "Hang loose", "Moderate", "Slow down and unwind", "Wind down", "Take it easy, don't be so tense" and "Become less tense".)