Previously, during the period of gaining ground (2,7)
I believe the answer is:
in advance
'previously' is the definition.
Both the answer and definition are adverbs. Perhaps there's an association between them I don't understand?
'during the period of gaining ground' is the wordplay.
I cannot really see how this works, but
'during' could be 'in' (e.g. during February means in February) and 'in' is found within the answer.
'period' could be 'ad' (AD is an example) and 'ad' is found in the answer.
The remaining letters 'vance' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for in advance that I've seen before include "Beforehand - can invade accordingly (2.7)" , "Before the event" , "Ahead in place or time" , "early" .)