Try hard to find a vein (6)
I believe the answer is:
strain
I believe this clue is a double definition.
'try hard' is the first definition.
(thesaurus)
'a vein' is the second definition.
(thesaurus)
'to find' is the link.
(Other definitions for strain that I've seen before include "song" , "Exert oneself to the utmost" , "Trace" , "Put through a perforated device to separate" , "Race" .)