Period of relaxation having taking a knock largely with alien invasion (7)
I believe the answer is:
detente
'period of relaxation' is the definition.
(period of relaxed relations between enemies)
'taking a knock largely with alien invasion' is the wordplay.
I cannot really understand how this works, but
'alien' could be 'et' (extra terrestrial) and 'et' is present in the answer.
This may be the basis of clue (or it may be nonsense).
'having' is the link.
(Other definitions for detente that I've seen before include "Easing of hostility between countries" , "Easing of strained relationships between countries" , "Relaxed relations" , "Easing of hostility between nations" , "Slackening of tension" .)