Call for attention; troops straighten, to some extent (4)
I believe the answer is:
psst
'call for attention' is the definition.
(I've seen this in another clue)
'troops straighten to some extent' is the wordplay.
'to some extent' says the answer is hidden in the clue.
'PSST' is hidden in the letters of 'troops straighten'.
(Other definitions for psst that I've seen before include "Call to attract attention" , "Surreptitious attention-seeker" , "This is confidential" , "Sibilant sound to get someone's attention" , "(Whispered) may I have your attention?" .)