Principal in favour of some turning up on time (8)
I believe the answer is:
foremost
'principal' is the definition.
(I've seen this before)
'in favour of some turning up on time' is the wordplay.
'in favour of' becomes 'for' (being for a particular cause).
'turning up' says the letters should be written in reverse.
'on' says to put letters next to each other.
'time' becomes 't' (abbreviation).
'some' written backwards gives 'emos'.
'for'+'emos'+'t'='FOREMOST'
(Other definitions for foremost that I've seen before include "premier" , "First in time, place or order" , "Most prominent" , "First in time, place or importance" , "Principal" .)