Scientific study every NATO battalion keeps up (6)
I believe the answer is:
botany
'scientific study' is the definition.
(I've seen this before)
'every nato battalion keeps up' is the wordplay.
'keeps' says the answer is hidden in the clue.
'up' is a reversal indicator (in down clue: letters go upwards).
'ynatob' is hidden within 'every nato battalion'.
'ynatob' backwards is 'BOTANY'.
(Other definitions for botany that I've seen before include "plants growing in one area" , "nature study" , "The scientific study of plants" , "type of wool" , "Unlucky, doomed" .)