Thus on the way back, mother carries one heavy metal (6)
I believe the answer is:
osmium
'heavy metal' is the definition.
(I've seen this before)
'thus on the way back mother carries one' is the wordplay.
'thus' becomes 'so' (synonyms).
'on the way back' is a reversal indicator.
'mother' becomes 'mum' (informal term for mother).
'carries' indicates putting letters inside (inserted letters carried inside).
'one' becomes 'i' (Roman numeral).
'so' reversed gives 'os'.
'mum' placed around 'i' is 'mium'.
'os'+'mium'='OSMIUM'
(Other definitions for osmium that I've seen before include "Heavy metal" , "The heaviest metal known" , "Densest of all naturally occurring metallic elements, Os" , "Metallic element" , "Lustrous, extremely hard member of the platinum metals group" .)