Symbol of rank crack troops held at first (4)
I believe the answer is:
sash
'symbol of rank' is the definition.
'crack troops held at first' is the wordplay.
'crack troops' becomes 'sas' (Special Air Service).
'at first' indicates taking the first letters.
The first letter of 'held' is 'h'.
'sas'+'h'='SASH'
(Other definitions for sash that I've seen before include "Item of clothing - frame for glass" , "Broad band, the one me father wore?" , "Window frame - shoulder band" , "Sliding window frame" , "Framework holding window panes" .)