Jack, a possible runner in the heat (3)
I believe the answer is:
tar
'jack' is the definition.
(both are informal terms for sailor)
'a possible runner in the heat' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'runner' could be 'r' (abbreviation) and 'r' is found within the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tar that I've seen before include "Thick flammable liquid" , "Crewman" , "Informal sailor or road surfacing material" , "Informal sailor or road-surfacing material" , "Viscous bituminous substance" .)