Sustain attempts to avoid relegation (4,2)
I believe the answer is:
keep up
'sustain' is the definition.
(synonyms)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for keep up that I've seen before include "Match pace or store aloft" , "Maintain pace with - the Jones?" , "Progress at same pace as others, or maintain appearance" , "Continue to be in touch (with)" , "Maintain pace with, don't lag behind" .)