Specify putting giants' pants in river (9)
I believe the answer is:
designate
'specify' is the definition.
(synonyms)
'giants' pants in river' is the wordplay.
'pants' indicates anagramming the letters (UK informal term for 'nonsense' or 'rubbish').
'in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'giants' with letters rearranged gives 'signat'.
'signat' placed into 'dee' is 'DESIGNATE'.
'putting' is the link.
(Other definitions for designate that I've seen before include "Appointed but not yet in post" , "Assign" , "Allocate" , "Identify" , "Appoint" .)