Input device is crucial at home (6)
I believe the answer is:
keypad
'input device' is the definition.
(I know that keypad is a type of input device)
'crucial at home' is the wordplay.
'crucial' becomes 'key' (I've seen this before).
'at' means one lot of letters go next to another.
'home' becomes 'pad' (pad can mean a person's home).
'key'+'pad'='KEYPAD'
'is' acts as a link.
(Other definitions for keypad that I've seen before include "where characters get depressed" , "In control, buttoned up" , "Array of buttons on mobile phone" , "Mobile phone provides this" , "Data input device for computers etc" .)