Arresting Liberal Democrat, copper picked up investigation — there's only one way out of it (3,2,3)
I believe the answer is:
cul de sac
'there's only one way out of it' is the definition.
'arresting liberal democrat copper picked up investigation' is the wordplay.
'arresting' is an insertion indicator.
'liberal democrat' becomes 'ld' (common abbreviation used in UK politics).
'copper' becomes 'cu' (chemical symbol).
'picked up' says the letters should be written backwards.
'investigation' becomes 'case'.
'case' back-to-front is 'esac'.
'cu'+'esac'='cuesac'
'ld' inserted into 'cuesac' is 'CUL-DE-SAC'.
(Other definitions for cul de sac that I've seen before include "Dead-end road" , "Block end; no through road" , "Blind alley" , "Dead end street" .)