Perhaps Y-fronts of Uncle Neil keep nothing concealed these days (7)
I believe the answer is:
unknown
'perhaps y' is the definition.
(unknown in algebra)
'fronts of uncle neil keep nothing concealed these days' is the wordplay.
'fronts of' says to take the initial letters.
'concealed' indicates putting letters inside.
'these days' becomes 'now' (at this point in time).
The initial letters of 'uncle neil keep nothing' is 'unkn'.
'unkn' enclosing 'now' is 'UNKNOWN'.
(Other definitions for unknown that I've seen before include "Strange - obscure" , "Unidentified, strange" , "anon?" , "Anonymous" , "Quite obscure - like x" .)