Spot with exceptional perception an unknown character (4)
I believe the answer is:
espy
'spot' is the definition.
(synonyms)
'with exceptional perception an unknown character' is the wordplay.
'with' says to put letters next to each other.
'exceptional perception' becomes 'ESP' (short for extra-sensory perception).
'an unknown character' becomes 'y' (algebra).
'esp' next to 'y' is 'ESPY'.
(Other definitions for espy that I've seen before include "Perceive" , "Pick out at a distance" , "Spot, notice" , "Observe (arch.)" , "Catch sight of (poetic)" .)