Pastime of six, thoroughly discontented after start of play (8)
I believe the answer is:
activity
'pastime' is the definition.
(I've seen this in another clue)
'six thoroughly discontented after start of play' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'discontented' says to hollow out the word (remove centre letters) (the word's contents are removed).
'after' says to put letters next to each other.
'start of play' becomes 'act i' (Act I).
'thoroughly' with its middle taken out is 'ty'.
'vi'+'ty'='vity'
'vity' put after 'acti' is 'ACTIVITY'.
'of' acts as a link.
(Other definitions for activity that I've seen before include "Busyness" , "Something performed for recreation or exercise" , "Occupation or exercise" , "Recreational pursuit" , "Hobby, pastime" .)