Absurdly, he preselects a competitor in race (13)
I believe the answer is:
steeplechaser
'competitor in race' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'absurdly he preselects a' is the wordplay.
'absurdly' indicates an anagram (I've seen this in other clues).
'he'+'preselects'+'a'='hepreselectsa'
'hepreselectsa' is an anagram of 'STEEPLECHASER'.
Can you help me to learn more?
(Other definitions for steeplechaser that I've seen before include "Horse in Grand National, for example" , "Racehorse; 13" , "Racing horse" , "Competitive runner" , "Horse for jump races" .)