Thickness of new red bath (7)
I believe the answer is:
breadth
'thickness' is the definition.
(thesaurus)
'new red bath' is the wordplay.
'new' indicates anagramming the letters.
'red'+'bath'='redbath'
'redbath' anagrammed gives 'BREADTH'.
'of' acts as a link.
(Other definitions for breadth that I've seen before include "Liberal view" , "Lateral extent" , "Wide measurement" , "Wideness" , "Measure" .)