Attempt to obtain one pound hat (6)
I believe the answer is:
trilby
'hat' is the definition.
(trilby is a kind of hat)
'attempt to obtain one pound' is the wordplay.
'attempt' becomes 'try' (synonyms).
'to obtain' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'pound' becomes 'lb' (synonyms).
'i'+'lb'='ilb'
'try' going around 'ilb' is 'TRILBY'.
(Other definitions for trilby that I've seen before include "kind of headgear" , "one by du Maurier?" , "Style of hat" , "one mesmerised" , "Man's soft felt hat" .)