Review of gene pool essentially was freely accessible (4)
I believe the answer is:
open
'freely accessible' is the definition.
(I've seen this before)
'review of gene pool essentially' is the wordplay.
'review of' says the letters should be written backwards.
'essentially' indicates the answer is hidden within the clue (essence can mean the inside of something).
'nepo' is hidden in the letters of 'gene pool'.
'nepo' reversed gives 'OPEN'.
'was' acts as a link.
(Other definitions for open that I've seen before include "Unfasten" , "such a verdict?" , "Undo; exposed to attack" , "Accessible - candid" , "Not closed or obstructed" .)