Large pole's covered in lead (4)
I believe the answer is:
clue
'lead' is the definition.
(a lead is a clue in an investigation)
'large pole's covered' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'pole's' becomes 'cue' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'covered' means one lot of letters goes inside another.
'l' put within 'cue' is 'CLUE'.
'in' is the link.
Can you help me to learn more?
(Other definitions for clue that I've seen before include "Crossword hint?" , "It helps towards the solution" , "Hint, tip" , "Crossword aid" , "Crossword help" .)