Heads across the London area to collect book (7)
I believe the answer is:
obverse
'heads' is the definition.
(I've seen this before)
'across the london area to collect book' is the wordplay.
'across' becomes 'over' (synonyms).
'the london area' becomes 'SE' (South East postcode).
'to collect' is an insertion indicator (inserted letters are collected by others).
'book' becomes 'b' (abbreviation for book).
'over'+'se'='overse'
'overse' enclosing 'b' is 'OBVERSE'.
(Other definitions for obverse that I've seen before include "One side of a coin" , "Side of a coin that shows the head" , "Penny may present such a face" , "Side of coin that bears principal design or head" , "Verbose on the opposite side" .)