Attempt to keep one pound in hat (6)
I believe the answer is:
trilby
'hat' is the definition.
(trilby is a kind of hat)
'attempt to keep one pound' is the wordplay.
'attempt' becomes 'try' (synonyms).
'to keep' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'pound' becomes 'lb' (synonyms).
'i'+'lb'='ilb'
'try' placed around 'ilb' is 'TRILBY'.
'in' is the link.
(Other definitions for trilby that I've seen before include "Felt hat with a creased crown" , "kind of headgear" , "du Maurier's heroine" , "Hat - George du Maurier heroine" , "Soft hat with indented crown" .)