Lecture companion that is pinching diamonds (5)
I believe the answer is:
chide
'lecture' is the definition.
(I've seen this before)
'companion that is pinching diamonds' is the wordplay.
'companion' becomes 'ch' (Companion of Honour).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'pinching' means one lot of letters goes inside another (taking inside).
'diamonds' becomes 'd' (abbreviation in card games).
'ie' enclosing 'd' is 'ide'.
'ch'+'ide'='CHIDE'
(Other definitions for chide that I've seen before include "Tear off strip" , "Reprove" , "Santa's laugh" , "Rebuke, scold" , "Censure angrily" .)