Understand Greek as poetry primarily (5)
I believe the answer is:
grasp
'understand' is the definition.
(grasping is a kind of understanding)
'greek as poetry primarily' is the wordplay.
'greek' becomes 'gr'.
'primarily' indicates taking the first letters.
The first letter of 'poetry' is 'p'.
'gr'+'as'+'p'='GRASP'
(Other definitions for grasp that I've seen before include "Follow - understand - seize" , "Take hold of firmly" , "Seize - comprehend" , "Support; rugby player" , "Comprehension" .)