English poised to alter instalment (7)
I believe the answer is:
episode
'instalment' is the definition.
(synonyms)
'english poised to alter' is the wordplay.
'english' becomes 'E' (abbreviation).
'to alter' indicates anagramming the letters.
'poised' with letters rearranged gives 'pisode'.
'e'+'pisode'='EPISODE'
(Other definitions for episode that I've seen before include "Instalment of a TV series" , "Instalment of broadcast serial" , "Programme of a serial" , "Instalment; event" , "One programme of a series" .)