Sun’s effect on fellow, touching (7)
I believe the answer is:
tangent
'touching' is the definition.
(a tangent touches on a circle)
'sun's effect on fellow' is the wordplay.
'sun's effect' becomes 'tan' (suntan).
'on' is a charade indicator (letters next to each other).
'fellow' becomes 'gent' (synonyms).
'tan'+'gent'='TANGENT'
(Other definitions for tangent that I've seen before include "What people often go off at!" , "At which one can go off" , "Particular line" , "Line that touches curve without crossing it" , "Completely different line of thought - trigonometrical function" .)