Observe the fifth letter would be wrong (4)
I believe the answer is:
note
'observe' is the definition.
(I know that observe can be written as note)
'fifth letter would be wrong' is the wordplay.
I cannot really understand how this works, but
'letter' could be 't' and 't' is located in the answer.
'wrong' could be 'no' ('no' can mean what just someone said is wrong) and 'no' is located in the answer.
A single letter 'e' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'the' acts as a link.
Can you help me to learn more?
(Other definitions for note that I've seen before include "Heed; record" , "Quaver, say" , "Make a mental or written reminder" , "A, B or C, for example - short message" , "Memo - sound - fame" .)