Southern harbour reduced by small amount (6)
I believe the answer is:
shaven
'reduced by small amount' is the definition.
(with a small amount shaved off)
'southern harbour' is the wordplay.
'southern' becomes 's' (abbreviation).
'harbour' becomes 'haven' (synonyms).
's'+'haven'='SHAVEN'
(Other definitions for shaven that I've seen before include "without locks" , "Hairless after work with razor" , "Without a beard" , "With hair removed (N, not D)" , "lacking coverage" .)