Attempt from left to shell stronghold (6)
I believe the answer is:
effort
'attempt' is the definition.
(I know this)
'left to shell stronghold' is the wordplay.
'left' shows that the letters should be reversed in order.
'to shell' becomes 'fe' (I can't explain this - if you can you should give a lot more credence to this answer).
'stronghold' becomes 'fort' (I've seen this in another clue).
'fe' written backwards gives 'ef'.
'ef'+'fort'='EFFORT'
'from' acts as a link.
Can you help me to learn more?
(Other definitions for effort that I've seen before include "Struggle" , "Attempt, energy exerted" , "Toil" , "Strain" , "For eft (anag.)" .)