The sky (9)
'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" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">the sky</span>' is the definition.<br/ ><span class="explanation-format__explanation">(the firmament means the sky)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for firmament that I've seen before include "Heavens, sky" , "The celestial sphere" , "Sky (literary)" , "The starry heaven" , "outer space, encapsulated" .)