Opened up, as one did when about to be late (7)
I believe the answer is:
dilated
'opened up' is the definition.
(I've seen this before)
'one did when about to be late' is the wordplay.
'when about' is an insertion indicator.
'to be late' becomes 'late'.
'did' placed around 'late' is 'DILATED'.
'as' acts as a link.
(Other definitions for dilated that I've seen before include "like pupils in the dark" , "Became more open" , "Spoke at length" , "Made more open/wide" , "Distended" .)