Become the thing to retire? (4,2)
I believe the answer is:
turn in
'retire?' is the definition.
(I know that bed can be written as turn in)
'become the thing' is the wordplay.
'become' becomes 'turn' (synonyms).
'the thing' becomes 'in' (I can't explain this - if you can you should give a lot more credence to this answer).
'turn'+'in'='TURN-IN'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for turn in that I've seen before include "Retire to bed" , "Hand over - retire" , "Go to bed, changing direction" , "Go to bed (informal)" , "rat on" .)