Then assume one hundred will turn up for the performance (7)
I believe the answer is:
recital
'the performance' is the definition.
(recital is a kind of performance)
'then assume one hundred will turn up' is the wordplay.
I cannot quite see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
'hundred' could be 'a' (synonyms) and 'a' is found within the answer.
'turn' could be 't' (abbreviation in Oxford Dictionary of Abbreviations) and 't' is found in the answer.
This explanation may well be incorrect...
'for' acts as a link.
Can you help me to learn more?
(Other definitions for recital that I've seen before include "statement of facts" , "Article found in performance" , "Concert by soloist" , "Public performance of music" , "Concert performed by a solo musician" .)