Mark brought us in to give support (7)
I believe the answer is:
'to give support' is the definition.
'mark brought us' is the wordplay.
I cannot really understand how this works, but
'mark' could be 'stain' (synonyms) and 'stain' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'in' is the link.
Can you help me to learn more?
(Other definitions for sustain that I've seen before include "Bear, hold, up" , "Undergo" , "Endure" , "Withstand" , "Maintain" .)