Nurse fits in coil, say, rejecting all potential for breeding? (4,4)
I believe the answer is:
gene pool
'all potential for breeding?' is the definition.
(all the potential genes in a population)
'nurse fits in coil say rejecting' is the wordplay.
'nurse' becomes 'en' (abbreviation for enrolled nurse).
'fits in' indicates putting letters inside.
'coil' becomes 'loop' (synonyms).
'say' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'rejecting' says the letters should be written backwards.
'loop'+'eg'='loopeg'
'en' going within 'loopeg' is 'loopeneg'.
'loopeneg' backwards is 'GENE POOL'.
(Other definitions for gene pool that I've seen before include "breeding stock" , "Sum of units of heredity in interbreeding population" , "All the DNA in a population" .)