Understood it's not done to jump on the bed (8,2)

'understood' is the definition.
(to cotton on to something is to understand it)

'not done to jump on the bed' is the wordplay.
'to jump' indicates an anagram (I've seen 'jumping' mean this).
'on' means one lot of letters go next to another.
'the bed' becomes 'cot' (cot is a type of bed).
'not'+'done'='notdone'
'notdone' anagrammed gives 'tonedon'.
'tonedon' put after 'cot' is 'COTTONED ON'.

'it's' acts as a link.