Orange squash on top of a herb (7)
I believe the answer is:
oregano
'a herb' is the definition.
(oregano is a kind of herb)
'orange squash on top' is the wordplay.
'squash' indicates an anagram (I've seen this in other clues).
'top' suggests taking the first letters (in a down clue, the first letter is top).
The initial letter of 'on' is 'o'.
'orange' with letters rearranged gives 'oregan'.
'oregan'+'o'='OREGANO'
'of' is the link.
(Other definitions for oregano that I've seen before include "part of Mediterranean diet?" , "Pungent seasoning leaves" , "Herb - a no-goer (anag)" , "Kitchen herb" , "Gear Ono up for the herb" .)