Travel day after day in desire for rare commodity (4,4)
I believe the answer is:
gold dust
'rare commodity' is the definition.
(a rare commodity might be described as gold dust)
'travel day after day in desire' is the wordplay.
'travel' becomes 'go' (synonyms).
'day after day' becomes 'DD' (two abbreviations for day).
'in' indicates putting letters inside.
'desire' becomes 'lust' (lusting is a kind of desiring).
'dd' placed into 'lust' is 'lddust'.
'go'+'lddust'='GOLD-DUST'
'for' acts as a link.
(Other definitions for gold dust that I've seen before include "Fine powder of precious metal" , "Something valuable (and very rare?)" , "something much sought-after" , "Proverbially rare stuff" .)