The commander came across one of the fictional reindeer (5)
I believe the answer is:
comet
'one of the fictional reindeer' is the definition.
'comet' can be an answer for 'reindeer' (name of one of Santa Claus reindeer). I'm not sure about the 'one of the fictional' bit.
'the commander came across' is the wordplay.
'the commander' becomes 'co'.
'came across' becomes 'met' (to come across is to meet or encounter).
'co'+'met'='COMET'
Can you help me to learn more?
(Other definitions for comet that I've seen before include "potential space invader" , "Tailed heavenly body" , "bright star" , "traveller covering huge mileage" , "It orbits the sun, with bright centre and long tail" .)