Without getting involved, sympathises with the party guy taking Rover
I believe the answer is:
fellow traveller
'without getting involved sympathises' is the definition.
I can't tell whether this defines the answer.
'the party guy taking rover' is the wordplay.
'the party guy' becomes 'fellow' (I am not sure about this - if you are sure you should believe this answer much more).
'taking' is a charade indicator (letters next to each other).
'rover' becomes 'traveller' (I've seen this before).
'fellow'+'traveller'='FELLOW-TRAVELLER'
'with' acts as a link.
Can you help me to learn more?
(Other definitions for fellow traveller that I've seen before include "Like monk for nun in Chaucer, for example" , "possibly 18 [PARTISAN]" , "one with leftist leanings" , "Communist sympathiser" , "political ally" .)