Strengthen a couple (5)
I believe the answer is:
brace
This clue is a double definition.
'strengthen' is the first definition.
(bracing is a kind of strengthening)
'a couple' is the second definition.
('brace' can be a synonym of 'couple')
(Other definitions for brace that I've seen before include "Support - couple" , "A pair as of game birds" , "Strengthener; pair" , "Steady; pair" , "Support; two" .)