Guy to speak about Tesla (4)
I believe the answer is:
stay
'guy' is the definition.
(guy can mean to steady or stay with rope)
'speak about tesla' is the wordplay.
'speak' becomes 'say' (I've seen this in other clues).
'about' means one lot of letters goes inside another.
'tesla' becomes 'T' (scientific unit).
'say' going around 't' is 'STAY'.
'to' is the link.
(Other definitions for stay that I've seen before include "Abide" , "Temporary residence" , "Supporting rope; remain" , "Delay (execution)" , "Sojourn" .)