Shadow old Republican worker in Savile Row (6)
I believe the answer is:
tailor
'worker in savile row' is the definition.
'shadow old republican' is the wordplay.
'shadow' becomes 'tail' (tailing is a kind of shadowing**).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'republican' becomes 'r' (abbreviation).
'tail'+'o'+'r'='TAILOR'
(Other definitions for tailor that I've seen before include "One who makes garments" , "Suit maker" , "Maker of clothes, men's mostly" , "Maker of men's suits" , "One makes suits etc. to measure" .)