Endeavour to secure one pound for a hat (6)
I believe the answer is:
trilby
'a hat' is the definition.
(trilby is a kind of hat)
'endeavour to secure one pound' is the wordplay.
'endeavour' becomes 'try' ('try' can be a synonym of 'endeavour').
'to secure' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'pound' becomes 'lb' ('lb' can be a synonym of 'pound').
'i'+'lb'='ilb'
'try' placed around 'ilb' is 'TRILBY'.
'for' acts as a link.
(Other definitions for trilby that I've seen before include "Soft hat with indented crown" , "Felt hat with a creased crown" , "Man's hat or girl hypnotised by Svengali" , "Indented felt hat" , "Hat - George du Maurier heroine" .)