Was first to support doe's mate that's collapsed (7)
I believe the answer is:
buckled
'collapsed' is the definition.
(buckling is a kind of collapsing)
'was first to support doe's mate' is the wordplay.
'was first' becomes 'led' (to lead is to go first).
'to support' says to put letters next to each other (in a down clue, some letters hold others up).
'doe's mate' becomes 'buck' (I've seen this before).
'led' put after 'buck' is 'BUCKLED'.
'that's' is the link.
(Other definitions for buckled that I've seen before include "collapsed" , "folded under pressure" , "Securely fastened" , "Fastened up - gave way" , "Crumpled under pressure." .)