Human say makes offer outside gym (5)
I believe the answer is:
biped
'human say' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'offer outside gym' is the wordplay.
'offer' becomes 'bid' (synonyms).
'outside' indicates putting letters inside.
'gym' becomes 'pe' (short for Physical Education).
'bid' enclosing 'pe' is 'BIPED'.
'makes' is the link.
Can you help me to learn more?
(Other definitions for biped that I've seen before include "Girl, say" , "Eg man" , "One walking on two legs" , "Two-legged animal" , "Using two pins" .)