A bit of a nightmare can spoil things (3)
I believe the answer is:
mar
'spoil things' is the definition.
'mar' can be an answer for 'spoil' ('mar' can be a synonym of 'spoil'). I am not sure about the 'things' bit.
'a bit of a nightmare can' is the wordplay.
'a bit of' indicates a hidden word (I've seen 'a bit' mean this (the answer is formed by a bit of the clue)).
'MAR' is hidden in the letters of 'nightmare can'.
Can you help me to learn more?
(Other definitions for mar that I've seen before include "Oh dear!" , "'Damage, impair (3)'" , "Spanish main?" , "Ruin" , "when spring growth starts?" .)