Character! He's skilled at hiding (3)
I believe the answer is:
ess
'character' is the definition.
(the letter S)
'he's skilled at hiding' is the wordplay.
'hiding' indicates the answer is hidden within the clue.
'ESS' is hidden within 'hes skilled at'.
(Other definitions for ess that I've seen before include "Alphabet letter" , "Curvaceous letter" , "Nineteenth letter" , "top secret" , "One next to tee" .)