Endeavour shown by office worker in a series of races (7)
I believe the answer is:
attempt
'endeavour' is the definition.
('attempt' can be a synonym of 'endeavour')
'office worker in a series of races' is the wordplay.
'office worker' becomes 'temp' (I've seen this before).
'in' means one lot of letters goes inside another.
'a series of races' becomes 'att' (I've seen this in other clues).
'temp' inserted inside 'att' is 'ATTEMPT'.
'shown by' acts as a link.
(Other definitions for attempt that I've seen before include "bid to kill someone" , "Try, hazard" , "Shot" , "Undertake" , "Have a crack" .)