Caviar expensive, it's said? It may be a buck (3,4)
I believe the answer is:
roe deer
'a buck' is the definition.
(thesaurus)
'caviar expensive it's said?' is the wordplay.
'caviar' becomes 'roe' (fish eggs).
'expensive' becomes 'dear' (similar in meaning).
'it's said?' indicates a 'sounds like' (homophone) clue.
'dear' is a homophone of 'deer'.
'roe'+'deer'='ROE-DEER'
'it may be' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for roe deer that I've seen before include "Small species of ungulate" , "Small (antlered) mammal" , "Countryside mammal" , "Brute" , "Delicate creature" .)