Merchant at first devious has to promise to give change (13)
I believe the answer is:
metamorphosis
'change' is the definition.
(metamorphosis is a kind of change)
'merchant at first devious has to promise to' is the wordplay.
'at first' indicates taking the first letters.
'devious' is an anagram indicator.
'to' says to put letters next to each other (I've seen this in other clues).
The initial letter of 'merchant' is 'm'.
'promise'+'to'='promiseto'
'has' after 'promiseto' is 'promisetohas'.
'promisetohas' is an anagram of 'etamorphosis'.
'm'+'etamorphosis'='METAMORPHOSIS'
'give' is the link.
(Other definitions for metamorphosis that I've seen before include "changing?" , "Total change in form" , "Process of transformation" , "Complete change of appearance" , "Transmutation" .)