When finally very keen to get back in bed (5)
I believe the answer is:
divan
'bed' is the definition.
(I've seen this before)
'when finally very keen to get back' is the wordplay.
'finally' says to take the final letters.
'very keen' becomes 'avid' (I've seen this before).
'to get back' says the letters should be written backwards (I've seen 'got back' mean this).
The last letter of 'when' is 'n'.
'n'+'avid'='navid'
'navid' backwards is 'DIVAN'.
'in' is the link.
(Other definitions for divan that I've seen before include "Sort of bed" , "Sofa - bed" , "Sideless, backless couch" , "Piece of furniture" , "resting place" .)