Part of cluster I left infertile (7)
I believe the answer is:
sterile
'infertile' is the definition.
('sterile' can be similar in meaning to 'infertile')
'part of cluster i left' is the wordplay.
'part of' indicates a hidden word.
'STERILE' is hidden in the letters of 'cluster i left'.
(Other definitions for sterile that I've seen before include "unable to reproduce results" , "Germ-free" , "Free from living micro-organisms, absolutely clean" , "Unable to produce young" , "Uninspired" .)