One fellow involved in arson harassed to such an extent (7)
I believe the answer is:
insofar
'to such an extent' is the definition.
I can't judge whether this definition defines the answer.
'one fellow involved in arson harassed' is the wordplay.
'one' becomes 'I' (Roman numeral).
'fellow' becomes 'f' (abbreviation - of a society etc.).
'involved in' means one lot of letters goes inside another.
'harassed' indicates anagramming the letters.
'arson' anagrammed gives 'nsoar'.
'f' inserted inside 'nsoar' is 'nsofar'.
'i'+'nsofar'='INSOFAR'
Can you help me to learn more?
(Other definitions for insofar that I've seen before include "... to the extent" , "as you'd expect after this word's been given" .)