Supports opposition inside up in the station (4)
I believe the answer is:
post
'station' is the definition.
(I know that station can be written as post)
'supports opposition inside up in' is the wordplay.
'in' indicates a hidden word.
'up' is a reversal indicator (in down clue: letters go upwards).
'tsop' is hidden within 'supports opposition inside'.
'tsop' written backwards gives 'POST'.
'the' acts as a link.
(Other definitions for post that I've seen before include "Letters and parcels; job" , "Stake; appointment" , "Spot the mail" , "Mail; wooden stake" , "Put a letter in the box - stop it" .)