Lorna’s personal obsession — act independently (2,4,3,5)
I believe the answer is:
do ones own thing
'act independently' is the definition.
(I've seen this before)
'lorna's personal obsession' is the wordplay.
'lorna's' becomes 'Doone's' (Lorna Doone is a novel by RD Blackmoore).
'personal' becomes 'own' (eg one's own possessions are personal possessions).
'obsession' becomes 'thing' (I've seen this before).
'doones'+'own'+'thing'='DO ONES OWN THING'
(Other definitions for do ones own thing that I've seen before include "behave naturally" , "act independently" , "Behave characteristically" .)