Occasionally, Thea picks up poem (5)
I believe the answer is:
haiku
'poem' is the definition.
(haiku is a kind of poem)
'occasionally thea picks up' is the wordplay.
'occasionally' means one should take alternating letters.
'thea'+'picks'+'up'='theapicksup'
The alternating letters of 'theapicksup' are 'HAIKU'.
(Other definitions for haiku that I've seen before include "Stylised Japanese poem of 17 syllables" , "Jap 17 syllable poem" , "Japanese 3-line poem of 17 syllables" , "Epigrammatic verse" , "Type of Japanese poem" .)