Detect unknown variable underlying sixth sense (4)
I believe the answer is:
espy
'detect' is the definition.
(I've seen this before)
'unknown variable underlying sixth sense' is the wordplay.
'unknown variable' becomes 'y' (algebra. I am not sure about the 'unknown' bit.).
'underlying' means one lot of letters go next to another (in a down clue, some letters lie under others).
'sixth sense' becomes 'esp' (I've seen this in another clue).
'y' put after 'esp' is 'ESPY'.
Can you help me to learn more?
(Other definitions for espy that I've seen before include "Spot, notice" , "Pick out at a distance" , "Catch sight of (poetic)" , "Behold" , "Spot at a distance" .)