Highest recommendation by a California wine area (4)
I believe the answer is:
napa
'california wine area' is the definition.
(I've seen this in another clue)
'highest recommendation by a' is the wordplay.
'highest recommendation' becomes 'nap' (I can't justify this - if you can you should give a lot more credence to this answer).
'by' says to put letters next to each other.
'nap'+'a'='NAPA'
Can you help me to learn more?
(Other definitions for napa that I've seen before include "Californian valley, famous for wine" , "soft skin" , "California wine valley" .)