Completely and remarkably weird niche trousers (5,4)
I believe the answer is:
every inch
'completely' is the definition.
(synonyms)
'remarkably weird niche trousers' is the wordplay.
'remarkably' becomes 'very' (to a considerable degree).
'weird' indicates an anagram.
'trousers' indicates putting letters inside (to trouser can mean to steal or take).
'niche' is an anagram of 'einch'.
'very' put within 'einch' is 'EVERY INCH'.
'and' acts as a link.
(Other definitions for every inch that I've seen before include "Wholly" , "completely" , "Absolutely!" , "Entirely" , "In all departments" .)