Gather what’s finally going spare (5)
I believe the answer is:
glean
'gather' is the definition.
(I've seen this before)
'finally going spare' is the wordplay.
'finally' suggests the final letters.
'spare' becomes 'lean' ('lean' can be similar in meaning to 'spare').
The last letter of 'going' is 'g'.
'g'+'lean'='GLEAN'
'what's' is the link.
(Other definitions for glean that I've seen before include "Gather remnants of corn" , "Gather scraps" , "Gather (grain) after a harvest" , "Gather (after reapers)" , "Gather (slowly, with difficulty)" .)