Lash around head of sea monster (5)
I believe the answer is:
beast
'monster' is the definition.
(I've seen this in another clue)
'lash around head of sea' is the wordplay.
'lash' becomes 'beat' (I can't explain this - if you can you should give a lot more credence to this answer).
'around' is an insertion indicator.
'head of' suggests taking the first letters.
The initial letter of 'sea' is 's'.
'beat' going around 's' is 'BEAST'.
Can you help me to learn more?
(Other definitions for beast that I've seen before include "beef?" , "Large animal -- objectionable person" , "Wild animal" , "Monster" , "Animal, creature" .)