I've made a mistake ignoring leader in military manoeuvres (3)
I believe the answer is:
ops
'military manoeuvres' is the definition.
(short for operations)
'i've made a mistake ignoring leader' is the wordplay.
'i've made a mistake' becomes 'oops' (oops may be said to acknowledge a mistake).
'ignoring leader' means to remove the first letter.
'oops' with its first letter taken off is 'OPS'.
'in' is the link.
(Other definitions for ops that I've seen before include "facelifts, perhaps" , "Military operations" , "medical procedures" , "Operations (abbrev.)" , "military activity" .)