Leader of advance party in trouble (3)
I believe the answer is:
ado
'trouble' is the definition.
(thesaurus)
'leader of advance party' is the wordplay.
'leader of' says to take the initial letters.
'party' becomes 'do' (do is a kind of party).
The first letter of 'advance' is 'a'.
'a'+'do'='ADO'
'in' is the link.
(Other definitions for ado that I've seen before include "Flap" , "Goings-on" , "Hoo-ha" , "flapping" , "stew is provided" .)