Prolong (7)
'prolong' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for stretch that I've seen before include "Bit of exercise" , "Make to go further" , "Exercise - period" , "Make (something) longer" , "sort of limousine" .)
(Other definitions for sustain that I've seen before include "Carry" , "Provide nourishment" , "Support, keep going" , "Bear (something's weight)" , "Nurish" .)
'prolong' is the definition.
(I know that extend is a type of prolong)
This is the entire clue.
(Other definitions for extend that I've seen before include "Stretch out over a distance or lengthen in time" , "Reach out, say the hand of friendship" , "Draw out" , "make offer?" , "Reach out or stretch" .)