Heads of the army caught knight in course of action (4)
I believe the answer is:
tack
'course of action' is the definition.
(I've seen this before)
'heads of the army caught knight' is the wordplay.
'heads of' suggests taking the first letters.
The first letters of 'the army caught knight' is 'TACK'.
'in' acts as a link.
(Other definitions for tack that I've seen before include "Change course < material" , "Alter course - riding gear" , "Staple - food" , "Food; nail" , "vulgar" .)