Instant calls blocked by new PM's predecessors (8)
I believe the answer is:
mornings
'pm's predecessors' is the definition.
(pm meaning afternoon)
'instant calls blocked by new' is the wordplay.
'instant' becomes 'mo'.
'calls' becomes 'rings' ('ring' can be a synonym of 'call'**).
'blocked by' means one lot of letters goes inside another (I've seen this in other clues).
'new' becomes 'n'.
'rings' going around 'n' is 'rnings'.
'mo'+'rnings'='MORNINGS'
(Other definitions for mornings that I've seen before include "Periods between dawn and noon" , "They're between dawn and noon" , "more than once when he should be getting up?" .)