Nancy's very small plait (5)
I believe the answer is:
tress
'plait' is the definition.
(I know that plait can be written as tress)
'nancy's very small' is the wordplay.
'nancy's very' becomes 'tres' ('very' in French - Nancy is a French city).
'small' becomes 's' (abbreviation - e.g. clothes size).
'tres'+'s'='TRESS'
(Other definitions for tress that I've seen before include "Curl - lock" , "Obsolete support" , "Switch" , "Long lock or plait of hair" , "Braid of hair" .)