Wreck or use loo, pushing the one in front back a bit (7)

'wreck' is the definition.
(thesaurus)

'back a bit' is the wordplay.
'back' becomes 'ruin' (I can't justify this - if you can you should believe this answer much more).
'a bit' becomes 'ate' ('bite' can be similar in meaning to 'eat').
'ruin'+'ate'='RUINATE'

'or use loo pushing the one in front' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.