Starts to 'accentuate the affirmative' gripping this poet (5)
I believe the answer is:
yeats
'this poet' is the definition.
(Yeats is an example)
'starts to accentuate the affirmative gripping' is the wordplay.
'starts to' indicates taking the first letters.
'affirmative' becomes 'yes' (affirmative answer word).
'gripping' means one lot of letters goes inside another.
The first letters of 'accentuate the' is 'at'.
'at' going within 'yes' is 'YEATS'.
(Other definitions for yeats that I've seen before include "he wrote" , "Irish poet and dramatist, d. 1939" , "Irish poet (The Winding Stair)" , "Poet William Butler or painter Jack B" , "W. B. -- , Irish poet/dramatist" .)