Reduce to ruins a submarine centre (3)
I believe the answer is:
mar
'centre' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'reduce to ruins a submarine' is the wordplay.
'reduce' indicates the answer is hidden within the clue (I've seen 'reduced' mean this (reducing the letters in the clue shows the answer)).
'MAR' can be found hidden inside 'ruins submarine'.
Can you help me to learn more?
(Other definitions for mar that I've seen before include "wreck" , "Spoil stuff" , "Ram into and spoil" , "A windy month" , "Cause harm" .)