Hold back when about to help (7)
I believe the answer is:
reserve
'hold back' is the definition.
(thesaurus)
'about to help' is the wordplay.
'about' becomes 're' (regarding**).
'to help' becomes 'serve' (serving is a kind of helping).
're'+'serve'='RESERVE'
'when' acts as a link.
(Other definitions for reserve that I've seen before include "Keep for someone - spare bod" , "ice" , "Conservation area" , "Keep for future use" , "some distance" .)