Line of troops in action (6)
I believe the answer is:
crease
'line' is the definition.
(synonyms)
'troops in action' is the wordplay.
'troops' becomes 'RE' (Royal Engineers army regiment).
'in' is an insertion indicator.
'action' becomes 'case' (synonyms).
're' going within 'case' is 'CREASE'.
'of' acts as a link.
(Other definitions for crease that I've seen before include "Fold in clothes" , "Fold - wrinkle" , "Dagger rarely" , "Fold, maybe unmeant" , "Wrinkle on the cricket pitch?" .)