Their hollow measure could be best? (7)
I believe the answer is:
trounce
'be best?' is the definition.
(I've seen this before)
'their hollow measure' is the wordplay.
'hollow' means to remove the middle letters.
'measure' becomes 'ounce' (ounce is a kind of measure).
'their' with its centre taken out is 'tr'.
'tr'+'ounce'='TROUNCE'
'could' is the link.
(Other definitions for trounce that I've seen before include "Clobber" , "Crush" , "Roundly defeat" , "Beat heavily" , "summons" .)