Officers measure portents (7)
I believe the answer is:
ensigns
'officers' is the definition.
(I've seen this before)
'measure portents' is the wordplay.
'measure' becomes 'en' (typographical measure).
'portents' becomes 'signs' (I've seen this before).
'en'+'signs'='ENSIGNS'
(Other definitions for ensigns that I've seen before include "officers? Of lower rank" , "Sensing (anag)" , "hauled up on deck?" , "Flags for naval officers" , "Old soldiers" .)