Fool in his appearance (3)
I believe the answer is:
sap
'fool' is the definition.
(I know that fool can be written as sap)
'in his appearance' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SAP' is hidden within 'his appearance'.
(Other definitions for sap that I've seen before include "Weak foolish person" , "Internal tree moisture" , "Enfeeble" , "Cosh; plant's juice" , "sucker" .)