Engineers growing weary and going to bed (8)
I believe the answer is:
'going to bed' is the definition.
('retire' can be a synonym of 'go to bed')
'engineers growing weary' is the wordplay.
'engineers' becomes 're' (abbreviation for Royal Engineers).
'growing weary' becomes 'tiring' ('tire' can be a synonym of 'grow weary').
'and' is the link.
(Other definitions for retiring that I've seen before include "Shunning contact with others", "Giving up work permanently", "stepping back", "Timid", "Leaving work - shy".)