One unable to fly delayed rival (7)
I believe the answer is:
emulate
'rival' is the definition.
(I've seen this before)
'one unable to fly delayed' is the wordplay.
'one unable to fly' becomes 'emu' (emus are flightless birds).
'delayed' becomes 'late' (running late).
'emu'+'late'='EMULATE'
(Other definitions for emulate that I've seen before include "Strive to equal or better" , "Follow in the footsteps of" , "Try to be equal or better than" , "Try to do as well as another" , "Contend with" .)