The sky? (5)
'the sky?' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for limit that I've seen before include "Point beyond which one should not go" , "Outer extent" , "Restrict - term" , "Breaking point" , "Restrict - boundary" .)
'the sky' is the definition.
(the firmament means the sky)
This is all the clue.
(Other definitions for firmament that I've seen before include "Heavens, sky" , "The celestial sphere" , "Sky (literary)" , "The starry heaven" , "outer space, encapsulated" .)