A nuke about to engulf large open space (5)
I believe the answer is:
plaza
'open space' is the definition.
(I've seen this before)
'a nuke about to engulf large' is the wordplay.
'nuke' becomes 'zap' (synonyms).
'about' shows that the letters should be reversed in order.
'to engulf' is an insertion indicator.
'large' becomes 'L' (eg in clothes sizes).
'a'+'zap'='azap'
'azap' backwards is 'paza'.
'paza' going around 'l' is 'PLAZA'.
(Other definitions for plaza that I've seen before include "open space" , "Town square (Sp.)" , "Open urban square or public area" , "Shopping mall" , "Open square in Spain" .)