Grind the grit, so more dense (7)
I believe the answer is:
tighter
'more dense' is the definition.
('tight' can be a synonym of 'dense')
'grind the grit' is the wordplay.
'grind' indicates an anagram (I've seen this in other clues).
'the'+'grit'='thegrit'
'thegrit' with letters rearranged gives 'TIGHTER'.
'so' is the link.
(Other definitions for tighter that I've seen before include "Meaner" , "more mean" , "Less loose" , "even more tense" , "More constricted" .)