Attempt to carry one pound in hat (6)
I believe the answer is:
trilby
'hat' is the definition.
(I know that trilby is a type of hat)
'attempt to carry one pound' is the wordplay.
'attempt to' becomes 'try' (I've seen this in other clues).
'carry' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'pound' becomes 'lb' (abbreviation for the unit of weight).
'i'+'lb'='ilb'
'try' enclosing 'ilb' is 'TRILBY'.
'in' is the link.
(Other definitions for trilby that I've seen before include "Indented felt hat" , "one mesmerised" , "Miss O'Ferrall" , "Hat - George du Maurier heroine" , "Man's hat or girl hypnotised by Svengali" .)