Grind and cut small number inside (5)
I believe the answer is:
gnash
'grind' is the definition.
(gnashing is a kind of grinding)
'cut small number inside' is the wordplay.
'cut' becomes 'gash' ('gash' can be a synonym of 'cut'**).
'small number' becomes 'n' (I've seen this in other clues).
'inside' is an insertion indicator.
'gash' placed around 'n' is 'GNASH'.
'and' is the link.
(Other definitions for gnash that I've seen before include "Strike one's teeth together in rage" , "Grind away" , "Grind (teeth) together" , "grinding action" , "Grind one's teeth" .)