A shrewd six-footer (3)
I believe the answer is:
fly
This clue might be a double definition.
'a shrewd' is the first definition.
(fly can mean shrewd or astute)
'six-footer' is the second definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
Can you help me to learn more?
(Other definitions for fly that I've seen before include "Soar; hurry" , "Take wing" , "needs to get away" , "Run away; smart (colloq.)" , "Dipterous insect" .)