Gay writing he contributed to book (6)
I believe the answer is:
blithe
'gay' is the definition.
(I've seen this before)
'writing he contributed to book' is the wordplay.
'writing' becomes 'lit' (abbreviation for literature).
'contributed to' is a charade indicator (letters next to each other).
'book' becomes 'b' (abbreviation for book).
'lit'+'he'='lithe'
'lithe' after 'b' is 'BLITHE'.
(Other definitions for blithe that I've seen before include "Shelley: 'Hail to thee . . . . . . spirit! Bird thou never wert'" , "Cheerful and carefree" , "Carefree and casual" , "Happy, without worries or cares" , "Happy, joyous" .)