Ad-lib sentence enthralling dramatist (5)
I believe the answer is:
ibsen
'dramatist' is the definition.
(I know that Ibsen is a dramatist)
'ad-lib sentence enthralling' is the wordplay.
'enthralling' says the answer is hidden in the clue (gripping the letters).
'IBSEN' is hidden in the letters of 'ad lib sentence'.
(Other definitions for ibsen that I've seen before include "Norwegian dramatist and poet" , "Linesman" , "Henrik -, dramatist from fourteen down (Hedda Gabler)" , "not one for 'All work and no play ...'" , "Norwegian dramatist, d. 1906" .)