First off, replace outside toilets and relax (4,5)
I believe the answer is:
hang loose
'relax' is the definition.
(I've seen this before)
'first off replace outside toilets' is the wordplay.
'first off' means to remove the first letter.
'replace' becomes 'change' (changing is a kind of replacing).
'outside' is an insertion indicator.
'toilets' becomes 'loos' (loo can informally mean a toilet).
'change' with its first letter removed is 'hange'.
'hange' placed around 'loos' is 'HANG LOOSE'.
'and' is the link.
(Another definition for hang loose that I've seen is " relax".)