Hurried north to protect most of the bases (6)
I believe the answer is:
depths
'bases' is the definition.
The answer and definition can be both related to thinking as well as being plural nouns.
Perhaps there's an association between them I don't understand?
'hurried north to protect most of the' is the wordplay.
'hurried' becomes 'sped' ('speed' can be a synonym of 'hurry').
'north' says the letters should be written backwards (in a down clue, letters go up or \'north\').
'to protect' indicates putting letters inside (inserted letters are protected or taken in).
'most of' means to remove the last letter.
'sped' back-to-front is 'deps'.
'the' with its last letter removed is 'th'.
'deps' enclosing 'th' is 'DEPTHS'.
Can you help me to learn more?
(Other definitions for depths that I've seen before include "obscurities" , "Deep water" , "Extents downward, backward or inward" , "Profundities, seas (fig.)" , "Lowest levels" .)