One of the Germans' U-boats (3)
I believe the answer is:
sub
'one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'of the germans' u-boats' is the wordplay.
'of' indicates a hidden word.
'SUB' is hidden in the letters of 'germans u boats'.
Can you help me to learn more?
(Other definitions for sub that I've seen before include "Warship; membership fee" , "Junior editor" , "Prefix meaning beneath" , "Beneath on bus" , "Briefly, membership fee or stand-in" .)