Think of a dumper, oddly (5,2)
I believe the answer is:
dream up
'think of' is the definition.
(synonyms)
'a dumper oddly' is the wordplay.
'oddly' indicates an anagram.
'a'+'dumper'='adumper'
'adumper' is an anagram of 'DREAM UP'.
(Other definitions for dream up that I've seen before include "Invent (something)" , "Imagine, invent" , "Concoct" , "'Devise or invent (5,2)'" , "Invent through imagination" .)