On foot, it's great! (3,3)
I believe the answer is:
big toe
'on foot it's great' is the definition.
I can't tell whether this definition defines the answer.
'on foot it's great' is the wordplay.
'on' means one lot of letters go next to another.
'foot' becomes 'toe' (toeing is a kind of footing).
'it's great' becomes 'big' (synonyms).
'toe' put after 'big' is 'BIG TOE'.
Can you help me to learn more?
(Other definitions for big toe that I've seen before include "Corn grows here" , "Largest foot digit" , "that once went to market?" , "member of the body" , "Largest pedal extremity" .)