Help with second letter, having misplaced the first (4)
I believe the answer is:
abet
'help' is the definition.
(thesaurus)
'with second letter having misplaced the first' is the wordplay.
'with' is an insertion indicator.
'second' becomes 'b' (B can mean second eg B-side).
'letter' becomes 'a'.
'having' says to put letters next to each other.
'misplaced the first' becomes 'et' (I can't explain this - if you can you should give a lot more credence to this answer).
'a'+'et'='aet'
'b' inserted within 'aet' is 'ABET'.
(Other definitions for abet that I've seen before include "Aid and help, perhaps in crime" , "Help in wrongdoing" , "Aid - connive with" , "Criminally endorse" , "Aid in offence" .)