Letter from occasionally sadistic husband (5)
I believe the answer is:
aitch
'letter' is the definition.
(aitch is the name for the letter H)
'occasionally sadistic husband' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
'husband' becomes 'h' (abbreviation).
The alternating letters of 'sadistic' are 'aitc'.
'aitc'+'h'='AITCH'
'from' is the link.
(Other definitions for aitch that I've seen before include "The letter h in full" , "Aspirated letter" , "that may be dropped" , "8th letter of the alphabet" , "bone" .)