First two of theatre seats, say, will do for cast (5)
I believe the answer is:
throw
'cast' is the definition.
(synonyms)
'first two of theatre seats say' is the wordplay.
'first two of theatre' becomes 'th' (I am not sure about this - if you are sure you should believe this answer much more).
'seats say' becomes 'row' (as in a row of seats in a venue).
'th'+'row'='THROW'
'will do for' is the link.
(Other definitions for throw that I've seen before include "Cast; give (party)" , "Discard" , "Try not to win" , "pitch or > floor" , "puzzle" .)