Try that Viagra, mostly while worried about sex (4,2,1,5)

'try' is the definition.
(giving it a whirl is a kind of trying)

'viagra mostly while worried about sex' is the wordplay.
'mostly' means to remove the last letter.
'worried' is an anagram indicator.
'about' means one lot of letters goes inside another.
'sex' becomes 'it' (a common euphemism).
'viagra' with its final letter taken off is 'viagr'.
'viagr'+'while'='viagrwhile'
'viagrwhile' anagrammed gives 'giveawhirl'.
'giveawhirl' placed around 'it' is 'GIVE IT A WHIRL'.

'that' acts as a link.