Old police company, climbing, come to pass (5)
I believe the answer is:
occur
'come to pass' is the definition.
(synonyms)
'old police company climbing' is the wordplay.
'old police' becomes 'RUC' (Royal Ulster Constabulary, former police force).
'company' becomes 'co' (abbreviation).
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'ruc'+'co'='rucco'
'rucco' reversed gives 'OCCUR'.
(Other definitions for occur that I've seen before include "Happen, transpire" , "Happen - to come to mind" , "exist" , "Befall" , "Spring to mind" .)