Time on strike, parking bum (5)
I believe the answer is:
tramp
'bum' is the definition.
('tramp' can be a synonym of 'bum')
'time on strike parking' is the wordplay.
'time' becomes 't'.
'on' says to put letters next to each other.
'strike' becomes 'ram' (I've seen this before).
'parking' becomes 'p'.
't'+'ram'+'p'='TRAMP'
(Other definitions for tramp that I've seen before include "cargo-boat" , "man down on his luck" , "Itinerant" , "Mendicant" , "one unlikely ever to have a mortgage!" .)