Engineers get careless and rest (5)
I believe the answer is:
relax
'rest' is the definition.
(I've seen this before)
'engineers get careless' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'get' is a charade indicator (letters next to each other).
'careless' becomes 'lax' (synonyms).
're'+'lax'='RELAX'
'and' acts as a link.
(Other definitions for relax that I've seen before include "Rest" , "Unbend" , "Don't worry" , "Put your feet up" , "Hang loose" .)