Gear soldiers packed in dismay (7)
I believe the answer is:
apparel
'gear' is the definition.
(I've seen this in another clue)
'soldiers packed in dismay' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'packed in' means one lot of letters goes inside another.
'dismay' becomes 'appal' (synonyms).
're' inserted into 'appal' is 'APPAREL'.
(Other definitions for apparel that I've seen before include "Attire" , "formal dress?" , "it's generally put on!" , "Clothing, things to wear" , "whatever people have on" .)