Imagine single girlfriend accepting sweetheart (6)
I believe the answer is:
ideate
'imagine' is the definition.
(I know that imagine can be written as ideate)
'single girlfriend accepting sweetheart' is the wordplay.
'single' becomes 'I' (one).
'girlfriend' becomes 'date' (I've seen this in other clues).
'accepting' means one lot of letters goes inside another (inserted letters are accepted).
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'date' enclosing 'e' is 'deate'.
'i'+'deate'='IDEATE'
(Other definitions for ideate that I've seen before include "Form a picture of" , "Imagine (psychology), aid tee (anag.)" , "Form in the imagination" , "Fancy" .)