Rely on crab moving a fraction of an inch (10)
I believe the answer is:
barleycorn
'a fraction of an inch' is the definition.
(historical unit of measurement)
'rely on crab moving' is the wordplay.
'moving' indicates anagramming the letters.
'rely'+'on'+'crab'='relyoncrab'
'relyoncrab' anagrammed gives 'BARLEYCORN'.
(Other definitions for barleycorn that I've seen before include "It was used to measure" , "Grain" .)