X is regressing, that's clear (3)
I believe the answer is:
net
'clear' is the definition.
(as in net profit)
'x is regressing' is the wordplay.
'x' becomes 'ten' (X is the Roman numeral for ten).
'is regressing' says the letters should be written backwards (to regress is to go backwards).
'ten' in reverse letter order is 'NET'.
'that's' acts as a link.
(Other definitions for net that I've seen before include "Mesh for the bottom line?" , "Mesh or web" , "Fish-catcher" , "Ultimate" , "Amount remaining after deductions" .)