Rely on expending energy in part of pool (6)
I believe the answer is:
depend
'rely' is the definition.
(to depend on someone is to rely on them)
'expending energy in part of pool' is the wordplay.
I cannot quite see how this works, but
'energy' could be 'pe' (P.E. is an example) and 'pe' is located in the answer.
'part' could be 'end' (end is a kind of part) and 'end' is present in the answer.
A single letter 'd' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'on' acts as a link.
(Other definitions for depend that I've seen before include "Rely (upon)" , "Put trust (in)" , "Rely or hang on" , "Rest" , "Dangle - rely" .)