Perhaps watch while undermined by resistance (5)
I believe the answer is:
timer
'perhaps watch' is the definition.
(a watch times things)
'while undermined by resistance' is the wordplay.
I cannot really see how this works, but
'while' could be 'time' (I've seen this before) and 'time' is present in the answer.
'resistance' could be 'r' (abbreviation in physics and electronics) and 'r' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for timer that I've seen before include "It measures intervals" , "Clock, say" , "hourglass" , "Pre-set switch" , "Eg, clock" .)