Expose rascal backing tall story about Republican (7)
I believe the answer is:
imperil
'expose' is the definition.
(I've seen this before)
'rascal backing tall story about republican' is the wordplay.
'rascal' becomes 'imp' (synonyms).
'backing' is a reversal indicator.
'tall story' becomes 'lie'.
'about' indicates putting letters inside.
'republican' becomes 'r' (abbreviation).
'lie' going around 'r' is 'lire'.
'lire' reversed gives 'eril'.
'imp'+'eril'='IMPERIL'
(Other definitions for imperil that I've seen before include "Jeopardize" , "Put in danger" , "Put at risk" , "Threaten, jeopardise" , "Endanger" .)