Platform Four's last at Old Street — strange (7)
I believe the answer is:
rostrum
'platform' is the definition.
(I know that rostrum is a type of platform)
'four's last at old street strange' is the wordplay.
'last' says to take the final letters.
'at' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'street' becomes 'st' (abbreviation).
'strange' becomes 'rum' (synonyms).
The final letter of 'four' is 'r'.
'r'+'o'+'st'+'rum'='ROSTRUM'
(Other definitions for rostrum that I've seen before include "stage" , "At which one may conduct" , "beak" , "Platform for orchestra conductor" , "Platform for speaker" .)