Put together our clothes, prepared to sign up (4,3,7)

'sign up' is the definition.
(to join the colours is to enlist in the army)

'put together our clothes prepared' is the wordplay.
'put together' becomes 'join' (joining is a kind of putting together).
'prepared' is an anagram indicator.
'our'+'clothes'='ourclothes'
'ourclothes' anagrammed gives 'thecolours'.
'join'+'thecolours'='JOIN THE COLOURS'

'to' acts as a link.