Fall onto bed finally, as weary (5)
I believe the answer is:
drain
'weary' is the definition.
(thesaurus)
'fall onto bed finally' is the wordplay.
'fall' becomes 'rain' (raining is a kind of falling**).
'onto' is a charade indicator (letters next to each other).
'finally' suggests the final letters.
The last letter of 'bed' is 'd'.
'rain' after 'd' is 'DRAIN'.
'as' is the link.
(Other definitions for drain that I've seen before include "Waste pipe" , "Make weak" , "Siphon off" , "Waste channel/pipe" , "Filter" .)