Cast seen in sixth row? Six missing (5)
I believe the answer is:
throw
'cast' is the definition.
(synonyms)
'sixth row? six missing' is the wordplay.
'missing' is a deletion indicator.
'sixth'+'row'='sixthrow'
'sixthrow' with 'six' taken out is 'THROW'.
'seen in' is the link.
(Other definitions for throw that I've seen before include "blanket" , "Cast, lob" , "Light rug" , "Try not to win" , "Hurl, fling" .)