One that may fight complaint from a person wearing tiny pants (8)
I believe the answer is:
antibody
'one that may fight complaint' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'a person wearing tiny pants' is the wordplay.
'person' becomes 'bod' (bod can informally mean a person or body).
'wearing' is an insertion indicator.
'pants' indicates anagramming the letters (UK informal term for 'nonsense' or 'rubbish').
'tiny' anagrammed gives 'ntiy'.
'bod' going inside 'ntiy' is 'ntibody'.
'a'+'ntibody'='ANTIBODY'
'from' acts as a link.
Can you help me to learn more?
(Other definitions for antibody that I've seen before include "protection against disease" , "Antigen attacker" , "Pathogen fighter" , "Fighter in blood" , "Protective protein in blood which destroys inimical substances" .)