Warrior tribe held back in reserve at first (5)
I believe the answer is:
iceni
'warrior tribe' is the definition.
'iceni' can be an answer for 'tribe' (I've seen this before). I'm not certain of the 'warrior' bit.
'held back in reserve at first' is the wordplay.
'held back' says the letters should be written backwards.
'reserve' becomes 'ice ' (both can mean aloofness).
'at first' says to put letters next to each other.
'in' in reverse letter order is 'ni'.
'ni' put after 'ice' is 'ICENI'.
Can you help me to learn more?
(Other definitions for iceni that I've seen before include "Rebellious tribe" , "Old warriors" , "British people once" , "Old tribe" , "Boudicca's tribe" .)