Doctor Ken with antigen for Thomas? (4,6)
I believe the answer is:
tank engine
'thomas?' is the definition.
(as in Thomas the Tank Engine)
'doctor ken with antigen' is the wordplay.
'doctor' indicates an anagram.
'with' means one lot of letters go next to another.
'ken' put after 'antigen' is 'antigenken'.
'antigenken' anagrammed gives 'TANK-ENGINE'.
'for' is the link.
(Other definitions for tank engine that I've seen before include "Possibly Thomas" , "Thomas, perhaps" , "army vehicle power unit" , "Thomas, say" .)