Rookie three feet ahead of girl (8)
I believe the answer is:
'rookie' is the definition.
Both the definition and answer are singular nouns.
Perhaps they are linked in a way I don't understand?
'three feet ahead of girl' is the wordplay.
'three feet' becomes 'yard' (I've seen this before).
'ahead of' means one lot of letters go next to another (I've seen this in other clues).
'girl' becomes 'bird' (bird is a kind of girl).
Can you help me to learn more?