It's more difficult holding one that's stronger (7)

'stronger' is the definition.
(hardy can mean fit or strong)

'it's more difficult holding one' is the wordplay.
'it's more difficult' becomes 'harder' ('hard' is associated in meaning with 'difficult').
'holding' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'harder' placed around 'i' is 'HARDIER'.

'that's' acts as a link.