Using bit of weed, lovemaking gets hard (4)
I believe the answer is:
with
'using' is the definition.
(eg 'fixed with glue' means 'fixed using glue')
'bit of weed lovemaking gets hard' is the wordplay.
'bit of' suggests taking the first letters.
'lovemaking' becomes 'it' (both are euphemisms for sex).
'gets' says to put letters next to each other.
'hard' becomes 'h' (abbreviation used in pencil classifications).
The first letter of 'weed' is 'w'.
'w'+'it'+'h'='WITH'
(Other definitions for with that I've seen before include "In the presence of" , "Alongside" , "Characterised by" , "Accompanying" , "In the company of" .)