That is to say, arranged help for arduous trek (6)
I believe the answer is:
schlep
'arduous trek' is the definition.
'that is to say arranged help' is the wordplay.
'that is to say' becomes 'sc' (abbrev for Latin scilicet meaning 'namely').
'arranged' is an anagram indicator.
'help' with letters rearranged gives 'hlep'.
'sc'+'hlep'='SCHLEP'
'for' acts as a link.
(Other definitions for schlep that I've seen before include "Tedious or difficult journey" , "Clumsy and stupid person" , "[TURKEY] -- > difficult procedure" , "clumsy individual" , "stupid and incompetent person" .)