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' is a reversal indicator.
'to engulf' indicates putting letters inside.
'large' becomes 'L' (eg in clothes sizes).
'a'+'zap'='azap'
'azap' reversed gives 'paza'.
'paza' enclosing '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" .)