Fashion initially marvellous computer device (5)
I believe the answer is:
modem
'computer device' is the definition.
(I've seen this before)
'fashion initially marvellous' is the wordplay.
'fashion' becomes 'mode' ('mode' can be a synonym of 'fashion'**).
'initially' indicates taking the first letters.
The first letter of 'marvellous' is 'm'.
'mode'+'m'='MODEM'
(Other definitions for modem that I've seen before include "Device for connecting two computers" , "Online connection device" , "Phone line and computer connector" , "Connector for computer to telephone line" , "Internet connection" .)