Develop over time (7)
I believe the answer is:
gestate
'develop over time' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gestate that I've seen before include "carry child" , "Carry in the womb" , "Evolve, ripen" , "Develop (idea, foetus)" , "Develop over time" .)