Checks Nadal's OK to be introduced to supporters (7)
I believe the answer is:
resists
'checks' is the definition.
The definition and answer can be both related to competing as well as being verbs in their -s form.
Maybe they are linked in a way I don't understand?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for resists that I've seen before include "Sisters (anag.)" , "Defends" , "Defies" , "Is hostile to" , "Struggles against" .)