Height of a cap? (4)
I believe the answer is:
peak
I believe this clue is a double definition.
'height' is the first definition.
(synonyms)
'a cap?' is the second definition.
(thesaurus)
'of' is the link.
(Other definitions for peak that I've seen before include "High point" , "Tip" , "Apex" , "Mountain top" , "Acme" .)