Son cunning about feet being fast (7)
I believe the answer is:
swiftly
'fast' is the definition.
(synonyms)
'son cunning about feet' is the wordplay.
'son' becomes 's'.
'cunning' becomes 'wily' (synonyms).
'about' means one lot of letters goes inside another.
'feet' becomes 'ft' (I've seen this in other clues).
'wily' placed around 'ft' is 'wiftly'.
's'+'wiftly'='SWIFTLY'
'being' acts as a link.
(Other definitions for swiftly that I've seen before include "Quickly" , "at speed" .)