Engineers get careless and rest (5)
I believe the answer is:
Here is my best explanation:
'rest' is the definition.
'engineers get careless' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'get' says to put letters next to each other.
'careless' becomes 'lax' ('lax' can be a synonym of 'careless').
'and' acts as a link.
(Other definitions for relax that I've seen before include "Hang loose", "Slow down and unwind", "Chill out", "put feet up", "Slacken", "Don't worry", "Put your feet up", "Wind down", "Loosen up, slow down" and "Rest".)