Confuse Serpent mostly without fail ultimately (5)
I believe the answer is:
addle
'confuse' is the definition.
(I know that addle is a more specific form of confuse)
'serpent mostly without fail ultimately' is the wordplay.
'serpent' becomes 'adder' (type of snake).
'mostly' means to remove the last letter (most but not all of the word).
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'fail ultimately' becomes 'l' (I can't explain this - if you can you should give a lot more credence to this answer).
'adder' with its last letter taken off is 'adde'.
'adde' going around 'l' is 'ADDLE'.
Can you help me to learn more?
(Other definitions for addle that I've seen before include "Befuddle" , "Get confused" , "Confuse; become rotten" , "confusing" , "Get rotten" .)