Legal action involving hard pursuit (5)
I believe the answer is:
chase
'pursuit' is the definition.
(synonyms)
'legal action involving hard' is the wordplay.
'legal action' becomes 'case' (a legal case).
'involving' is an insertion indicator.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'case' enclosing 'h' is 'CHASE'.
(Other definitions for chase that I've seen before include "Hound" , "Composed type accommodated in this" , "Pursue; decorate (metal) by engraving" , "Engrave - hunt - pursue" , "Pursuit" .)