Girl jokes when speaking in bed (6)
I believe the answer is:
matrix
'bed' is the definition.
(bed in a slab which brass is fixed in)
'girl jokes when speaking' is the wordplay.
I cannot really see how this works, but
'girl' could be 'trix' and 'trix' is present in the answer.
The remaining letters 'ma' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
'in' acts as a link.
(Other definitions for matrix that I've seen before include "Mathematical array of numbers" , "Mould; lattice" , "table" , "Array of numbers in rows and columns" , "Rectangular array of quantities or symbols" .)