Suffer to keep up (7)
I believe the answer is:
sustain
This clue is a double definition.
'suffer' is the first definition.
(I know that suffer can be written as sustain)
'keep up' is the second definition.
(I know that keep up can be written as sustain)
'to' acts as a link.
(Other definitions for sustain that I've seen before include "Withstand" , "Nurish" , "Keep alive" , "Provide nourishment" , "Bear (something's weight)" .)