Previously, during the period of gaining ground (2,7)
I believe the answer is:
in advance
'previously' is the definition.
Both the definition and answer are adverbs. Perhaps there's a link between them I don't understand?
'during the period of gaining ground' is the wordplay.
I cannot quite understand 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 within the answer.
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" .)