Right inside important jail (4)
I believe the answer is:
brig
'jail' is the definition.
(I've seen this in another clue)
'right inside important' is the wordplay.
'right' becomes 'r' (common abbreviation).
'inside' is an insertion indicator.
'important' becomes 'big' (similar in meaning).
'r' going within 'big' is 'BRIG'.
(Other definitions for brig that I've seen before include "Two-masted square rigger" , "staff officer briefly used" , "Two-master; army officer (abbr.)" , "Crew can" , "Two-masted square-rigger" .)