Open up head of big fish (6)
I believe the answer is:
broach
'open up' is the definition.
(I've seen this before)
'head of big fish' is the wordplay.
'head of big' becomes 'b' (I can't explain this - if you can you should believe this answer much more).
'fish' becomes 'roach' (roach is a kind of fish).
'b'+'roach'='BROACH'
Can you help me to learn more?
(Other definitions for broach that I've seen before include "Hint" , "Pierce (a cask)" , "crack" , "Start to speak" , "Start to talk about" .)