Be present when old police company returns (5)
I believe the answer is:
occur
'be present' is the definition.
(thesaurus)
'old police company returns' is the wordplay.
'old police' becomes 'RUC' (Royal Ulster Constabulary, former police force).
'company' becomes 'co' (abbreviation).
'returns' shows that the letters should be reversed in order.
'ruc'+'co'='rucco'
'rucco' in reverse letter order is 'OCCUR'.
'when' is the link.
(Other definitions for occur that I've seen before include "exist" , "Happen - to come to mind" , "Go on" , "Befall" , "Come to pass" .)