I'd written up poetry that's different (7)
I believe the answer is:
diverse
'different' is the definition.
(similar in meaning)
'i'd written up poetry' is the wordplay.
'written up' is a reversal indicator.
'poetry' becomes 'verse' (synonyms).
'id' in reverse letter order is 'di'.
'di'+'verse'='DIVERSE'
'that's' acts as a link.
(Other definitions for diverse that I've seen before include "Differing" , "Widely varied" , "Many and various" , "I served in many ways" , "Revised, being of different kinds" .)