Rival poet is performing work at the start (8)
I believe the answer is:
opposite
'rival' is the definition.
(on the opposite side)
'poet is performing work at the start' is the wordplay.
'performing' indicates an anagram.
'work' becomes 'op' (common abbreviation for 'opus').
'at the start' means one lot of letters go next to another.
'poet'+'is'='poetis'
'poetis' is an anagram of 'posite'.
'posite' after 'op' is 'OPPOSITE'.
(Other definitions for opposite that I've seen before include "Oppose it, being faced against" , "Corresponding" , "Diametrically contrary" , "Totally different; facing" , "Directly facing each other" .)