Filbert's getting on trousers at home, occupied with thoughts (9)
I believe the answer is:
imagining
'occupied with thoughts' is the definition.
(thinking or imagining things)
'filbert's getting on trousers at home' is the wordplay.
'filbert's' becomes 'I'm' (where the crossword setter is 'Filbert').
'getting on' becomes 'aging' (I've seen this before).
'trousers' is an insertion indicator (trouser can mean to steal or take).
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'aging' going around 'in' is 'agining'.
'im'+'agining'='IMAGINING'
(Other definitions for imagining that I've seen before include "Supposing, assuming" , "Fantasising" , "Thinking up" , "Fancying" , "illusion?" .)