Long snake about to eat iguana's head (6)
I believe the answer is:
aspire
'long' is the definition.
(I've seen this before)
'snake about to eat iguana's head' is the wordplay.
'snake' becomes 'asp' (asp is a kind of snake).
'about to' becomes 're' (I can't justify this - if you can you should give a lot more credence to this answer).
'eat' indicates putting letters inside.
'head' indicates taking the first letters.
The initial letter of 'iguanas' is 'i'.
'asp'+'re'='aspre'
'aspre' enclosing 'i' is 'ASPIRE'.
Can you help me to learn more?
(Other definitions for aspire that I've seen before include "Aim, have an ambitious plan" , "Have an ambitious plan or a lofty goal" , "Seek (to do something)" , "Long" , "Hope (to achieve)" .)