First of all merchants exchange limes or nectarines for other fruit (5)
I believe the answer is:
melon
'other fruit' is the definition.
'melon' can be an answer for 'fruit' (type of fruit). I am not sure about the 'other' bit.
'first of all merchants exchange limes or nectarines' is the wordplay.
'first of all' suggests taking the first letters.
The initial letters of 'merchants exchange limes or nectarines' is 'MELON'.
'for' is the link.
Can you help me to learn more?
(Other definitions for melon that I've seen before include "Type of fruit" , "Honeydew fruit" , "Sweet juicy fruit" , "Member of the gourd family" , "Edible gourd" .)