It helps to recall troops, having gathered round outside (7)
I believe the answer is:
memento
'it helps to recall' is the definition.
(item used to help remember something)
'troops having gathered round outside' is the wordplay.
'troops' becomes 'men' (I've seen this in another clue).
'having gathered' becomes 'met' (I can't explain this - if you can you should give a lot more credence to this answer).
'round' becomes 'o' (round can mean a round object).
'outside' indicates putting letters inside.
'met'+'o'='meto'
'men' put inside 'meto' is 'MEMENTO'.
Can you help me to learn more?
(Other definitions for memento that I've seen before include "might be kept for sentimental reasons" , "This reminds" , "Souvenir, something to remember by" , "Souvenir or token" , "Something kept" .)