Watches fight against welterweights, vacuously (5)
I believe the answer is:
views
'watches' is the definition.
('view' can be a synonym of 'watch')
'fight against welterweights vacuously' is the wordplay.
'fight' becomes 'vie' (I've seen this in other clues).
'against' says to put letters next to each other.
'vacuously' says to hollow out the word (remove centre letters).
'welterweights' with its centre removed is 'ws'.
'vie'+'ws'='VIEWS'
(Other definitions for views that I've seen before include "Opinions, perspectives" , "Aspects, panoramas" , "Vistas; opinions" , "Wives (anag.)" , "Looks at; opinions" .)