Hint — he leaves the running competition (5)
I believe the answer is:
trace
'hint' is the definition.
(synonyms)
'he leaves the running competition' is the wordplay.
'leaves' is a deletion indicator.
'running competition' becomes 'race' (race is a kind of competition. I am not sure about the 'running' bit.).
'the' with 'he' removed is 't'.
't'+'race'='TRACE'
Can you help me to learn more?
(Other definitions for trace that I've seen before include "Make a copy using transparent paper" , "Suggestion of rain" , "Barely detectable amount" , "Make a copy using semi-transparent paper" , "Track; small quantity" .)