Former alien queen in Devon city (6)
I believe the answer is:
exeter
'devon city' is the definition.
(I've seen this before)
'former alien queen' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'alien' becomes 'et' (extra terrestrial).
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'ex'+'et'+'er'='EXETER'
'in' acts as a link.
(Other definitions for exeter that I've seen before include "South-west English city" , "Oxford college; city" , "West Country cathedral city" , "South-western English city" , "The modern Isca Dumnoniorum" .)