Signal keeps Liberal in lead (4)
I believe the answer is:
clue
'lead' is the definition.
(a lead is a clue in an investigation)
'signal keeps liberal' is the wordplay.
'signal' becomes 'cue' (as in an actor's cue).
'keeps' is an insertion indicator (inserted letters are kept inside).
'liberal' becomes 'l' (abbreviation).
'cue' going around 'l' is 'CLUE'.
'in' is the link.
(Other definitions for clue that I've seen before include "Crossword help" , "Slight indication (as here)" , "Crossword aid" , "Evidence that helps to solve a problem" , "Aid to detection" .)