Last up on tour, singular London landmark (2,5)
I believe the answer is:
st pauls
'london landmark' is the definition.
(St Paul's Cathedral)
'last up on tour singular' is the wordplay.
'on' means one lot of letters go next to another.
'tour' indicates an anagram (I've seen 'tours' mean this (the letters roam or tour around)).
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'last'+'up'='lastup'
'lastup' with letters rearranged gives 'stpaul'.
'stpaul'+'s'='ST PAULS'
(Other definitions for st pauls that I've seen before include "Religious artefact" , "London cathedral" .)