Writer of compendium starts to get married to sitter (8)
I believe the answer is:
composer
'writer' is the definition.
(I've seen this before)
'compendium starts to get married to sitter' is the wordplay.
'starts to' indicates taking the first letters.
'get married to' becomes 'om' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sitter' becomes 'poser' (I've seen this before).
The first letter of 'compendium' is 'c'.
'c'+'om'+'poser'='COMPOSER'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for composer that I've seen before include "Mozart, e.g." , "One writes music" , "Wagner perhaps" , "Weill, Weber or Webber" , "Liszt or Ravel, say" .)