In double bed initially, diminutive nerd (5)
I believe the answer is:
dweeb
'nerd' is the definition.
(I've seen this before)
'in double bed initially diminutive' is the wordplay.
'in' means one lot of letters goes inside another.
'initially' says to take the initial letters.
'diminutive' becomes 'wee' (I've seen this in another clue).
The initial letters of 'double bed' is 'db'.
'db' placed around 'wee' is 'DWEEB'.
(Other definitions for dweeb that I've seen before include "silly American" , "Insignificant student" , "(US) idiot" , "Geek" , "anorak" .)