Rough argyle shirt added a lot (7)
I believe the answer is:
greatly
'a lot' is the definition.
(thesaurus)
'rough argyle shirt added' is the wordplay.
'rough' indicates an anagram (a rough or broken ordering of the letters).
'shirt' becomes 't' (as in a T-shirt).
'added' says to put letters next to each other.
'argyle'+'t'='argylet'
'argylet' anagrammed gives 'GREATLY'.
(Other definitions for greatly that I've seen before include "Very much" , "to a considerable extent" , "To a large extent" .)