At home, phoning about fellow trespassing (10)

'trespassing' is the definition.
(to infringe is to trespass or transgress)

'at home phoning about fellow' is the wordplay.
'at home' becomes 'in' (similar in meaning).
'phoning' becomes 'ringing' ('ring' can be a synonym of 'phone').
'about' is a charade indicator (letters next to each other).
'fellow' becomes 'F' (abbreviation - of a society etc.).
'ringing' after 'f' is 'fringing'.
'in'+'fringing'='INFRINGING'