Writer preens on odd occasions (3)
I believe the answer is:
pen
'writer' is the definition.
(something which writes)
'preens on odd occasions' is the wordplay.
'on odd occasions' indicates alternate letters (the odd-numbered letters are used).
The alternate letters of 'preens' are 'PEN'.
(Other definitions for pen that I've seen before include "Confine the writer" , "Female swan" , "'The ... is mightier than the sword'" , "See 20" , "Prison" .)