Seek interminably to understand (3)
I believe the answer is:
see
'understand' is the definition.
(synonyms)
'seek interminably' is the wordplay.
'interminably' means to remove the last letter.
'seek' with its last letter removed is 'SEE'.
'to' acts as a link.
(Other definitions for see that I've seen before include "around cathedral" , "Observe the bishop's domain" , "date" , "Norwich, for example" , "Perceive a bishopric" .)