Employed to secure luck, like a horseshoe (1-6)

'like a horseshoe' is the definition.
(horseshoes are U-shaped)

'employed to secure luck' is the wordplay.
'employed' becomes 'used' ('use' can be a synonym of 'employ').
'to secure' means one lot of letters goes inside another (inserted letters are secured inside).
'luck' becomes 'hap' (I've seen this in another clue).
'used' placed around 'hap' is 'U-SHAPED'.