Hard stone terrine regularly used (5)
I believe the answer is:
stern
'hard' is the definition.
(I've seen this in another clue)
'stone terrine regularly used' is the wordplay.
'stone' becomes 'st' (abbreviation).
'regularly used' means one should take alternating letters (letters selected on a regular basis).
The alternate letters of 'terrine' are 'ern'.
'st'+'ern'='STERN'
(Other definitions for stern that I've seen before include "Forbidding in aspect" , "Grimly serious; back" , "Unpleasantly serious" , "Strict, grimly serious" , "Rigid" .)