Top which may be lower (6)
I believe the answer is:
jersey
I believe this clue is a double definition.
'top' is the first definition.
(item of clothing)
'lower' is the second definition.
(type of cow, something which moos or lows)
'which may be' is the link.
(Other definitions for jersey that I've seen before include "whose yellow variety represents the cream" , "farm animal" , "Top - cow" , "Garment for upper body" , "lower?" .)