Frank looked perceptive (4-4)
I believe the answer is:
open-eyed
'perceptive' is the definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?
'frank looked' is the wordplay.
'frank' becomes 'open' (synonyms).
'looked' becomes 'eyed' (eyeing is a kind of looking).
'open'+'eyed'='OPEN-EYED'
Can you help me to learn more?
(Other definitions for open-eyed that I've seen before include "Surprised - aware" , "Forming an encrusted mass" , "Astonished; fully aware" , "Astonished - watchful" .)