Lean on ref once, injured (7)
I believe the answer is:
enforce
'lean on' is the definition.
(thesaurus)
'ref once injured' is the wordplay.
'injured' indicates anagramming the letters.
'ref'+'once'='refonce'
'refonce' with letters rearranged gives 'ENFORCE'.
(Other definitions for enforce that I've seen before include "Compel adherence to (a law)" , "Compel observation of" , "Compel or impose obedience, say to law" , "Ensure observance of rules" , "Put into effect" .)