Some length in the main chance (4)
I believe the answer is:
inch
'some length' is the definition.
(I've seen this before)
'in the main chance' is the wordplay.
'in' says the answer is hidden in the clue.
'INCH' is hidden within 'main chance'.
(Other definitions for inch that I've seen before include "Creep, edge" , "Creep towards < island" , "amount of rainfall?" , "0.08333 of a foot" , "Just a bit?" .)