Thus, on the way, we have an heir (3)
I believe the answer is:
son
'heir' is the definition.
(I've seen this in another clue)
'thus on the way' is the wordplay.
'thus' becomes 'so' (synonyms).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'the way' becomes 'n' (Compass direction - North).
'so'+'n'='SON'
'we have an' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for son that I've seen before include "Human offspring" , "Young boy" , "A relative" , "Male descendant" , "The kind of child" .)