A back number: that's clear (3)
I believe the answer is:
net
'clear' is the definition.
('net' can be a synonym of 'clear')
'a back number' is the wordplay.
'a back' is a reversal indicator.
'number' becomes 'ten' (ten is a kind of number).
'ten' in reverse letter order is 'NET'.
'that's' acts as a link.
(Other definitions for net that I've seen before include "It's see-through" , "Butterfly trap" , "land" , "catch fish" , "After taxes" .)