Theseus oddly appears in English river (4)
I believe the answer is:
tees
'english river' is the definition.
(I've seen this before)
'theseus oddly' is the wordplay.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
The alternate letters of 'theseus' are 'TEES'.
'appears in' is the link.
(Other definitions for tees that I've seen before include "Northern river" , "Flower" , "Middlesbrough's river" , "sand-hills" , "English river" .)