Engineers sit and rest (6)
I believe the answer is:
repose
'rest' is the definition.
('repose' can be a synonym of 'rest')
'engineers sit' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'sit' becomes 'pose' ('pose' can be a synonym of 'sit'**).
're'+'pose'='REPOSE'
'and' is the link.
(Other definitions for repose that I've seen before include "Lie at rest" , "Rest - tranquillity" , "take it easy" , "Quiet restfulness" , "calm" .)