Those possibly set in a row of four? (5)
I believe the answer is:
tines
'a row of four?' is the definition.
I can't tell whether this definition defines the answer.
'those possibly set in' is the wordplay.
'those' becomes 'in' (I can't justify this - if you can you should believe this answer much more).
'possibly' is an anagram indicator.
'in' means one lot of letters goes inside another.
'set' with letters rearranged gives 'tes'.
'in' inserted into 'tes' is 'TINES'.
Can you help me to learn more?
(Another definition for tines that I've seen is " Prongs of a fork".)