Feeling pain under military leader giving instruction (8)
I believe the answer is:
coaching
'instruction' is the definition.
(thesaurus)
'feeling pain under military leader' is the wordplay.
'feeling pain' becomes 'aching' (I've seen this before).
'under' says to put letters next to each other.
'military leader' becomes 'co'.
'aching' after 'co' is 'COACHING'.
'giving' acts as a link.
(Other definitions for coaching that I've seen before include "Training, teaching" , "Sports instructing or training" , "Tutoring" , "tuition" .)