Strengthen, support (7)
(Other definitions for bolster that I've seen before include "Large pillow" , "Lobster (anag.)" , "Extended pillow" , "support for head after retiring" , "Reinforce; a cushion" .)
I believe this clue is a double definition.
'strengthen' is the first definition.
(thesaurus)
'support' is the second definition.
(synonyms)
(Other definitions for sustain that I've seen before include "Withstand" , "Bear, hold, up" , "Carry" , "Endure" , "Support, keep going" .)