Slack police department going after learner driver in fact dropping back (7)
I believe the answer is:
flaccid
'slack' is the definition.
(I've seen this before)
'police department going after learner driver in fact dropping back' is the wordplay.
'police department' becomes 'cid' (I've seen this in other clues).
'going after' says to put letters next to each other.
'learner driver' becomes 'l' (I've seen this in other clues).
'in' indicates putting letters inside.
'dropping back' means to remove the last letter.
'fact' with its last letter removed is 'fac'.
'l' put within 'fac' is 'flac'.
'cid' put after 'flac' is 'FLACCID'.
(Other definitions for flaccid that I've seen before include "Floppy" , "Droopy" , "Lax" , "Slack" , "Limp and soft" .)