Concentrate on the nitty-gritty (7)
I believe the answer is:
essence
I believe this clue is a double definition.
'concentrate' is the first definition.
(I've seen this in another clue)
'the nitty-gritty' is the second definition.
(synonyms)
'on' is the link.
(Other definitions for essence that I've seen before include "Nitty-gritty" , "Defining quality" , "Indispensable" , "Concentrated liquid flavouring" , "Intrinsic nature; four Dn" .)