Strict, always, in extreme sense (6)
I believe the answer is:
severe
'strict' is the definition.
(I've seen this before)
'always in extreme sense' is the wordplay.
'always' becomes 'ever' (synonyms).
'in' means one lot of letters goes inside another.
'extreme' suggests removing the centre.
'sense' with its middle removed is 'se'.
'ever' inserted into 'se' is 'SEVERE'.
(Other definitions for severe that I've seen before include "Strait-laced" , "Extreme - strict" , "Very bad" , "Unsparing and uncompromising" , "'Strict, harsh (6)'" .)