Appearance that is evident in heartless fellow (4)
'appearance' is the definition.
(I've seen this before)
'that is evident in heartless fellow' is the wordplay.
'that is' becomes 'ie' (synonyms).
'evident in' indicates putting letters inside.
'heartless' means to remove the middle letters.
'fellow' becomes 'man' (I've seen this before).
'man' with its middle removed is 'mn'.
'ie' placed within 'mn' is 'MIEN'.
(Other definitions for mien that I've seen before include "attitude", "(One's) manner, bearing", "Counterance - mine?", "Air - look - manner", "person's look".)