In truth, this can be so monotonous! (3)
I believe the answer is:
rut
'in truth this can be so monotonous' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's a link between them I don't understand?
'in truth this' is the wordplay.
'in' indicates the answer is hidden within the clue.
'RUT' is hidden within 'truth this'.
Can you help me to learn more?
(Other definitions for rut that I've seen before include "Wheel-furrow" , "Furrow or groove as in track" , "does get followed during this" , "spell of arousal" , "Furrow left by wheel" .)