Jumping gun stops motor racing in kingdom (5)
I believe the answer is:
fungi
'kingdom' is the definition.
(I know that Fungi is a type of kingdom)
'jumping gun stops motor racing' is the wordplay.
'jumping' indicates anagramming the letters.
'stops' indicates putting letters inside.
'motor racing' becomes 'fi' (FI resembles F1, Formula 1).
'gun' anagrammed gives 'ung'.
'ung' put into 'fi' is 'FUNGI'.
'in' is the link.
(Other definitions for fungi that I've seen before include "chanterelles etc" , "Plants reproducing by spores" , "Spore-producing organisms" , "provides penicillin, etc" , "Kingdom of lower plants" .)