Carbon-free load in mythical ship (4)
I believe the answer is:
argo
'mythical ship' is the definition.
(I've seen this before)
'carbon-free load' is the wordplay.
'carbon' becomes 'C' (C is the chemical symbol for carbon).
'free' indicates named letters should be taken away.
'load' becomes 'cargo' (synonyms).
'cargo' with 'c' taken away is 'ARGO'.
'in' is the link.
(Other definitions for argo that I've seen before include "Ship Jason sailed to find the Golden Fleece" , "Jason's craft" , "Stars once recognised in the south" , "Agro about the constellation" , "Famous ship" .)