Prisoner associated with tower plot (8)
I believe the answer is:
conspire
'plot' is the definition.
(I know that conspire is a kind of plot)
'prisoner associated with tower' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'associated with' means one lot of letters go next to another.
'tower' becomes 'spire' (spire is a kind of tower).
'con'+'spire'='CONSPIRE'
(Other definitions for conspire that I've seen before include "Plot together - on prices (anag)" , "'Plot, connive (8)'" , "Secretly plan together" , "Collude" , "implicated in plot" .)