Send up regulation covering one police department making a comeback (8)
I believe the answer is:
ridicule
'send up' is the definition.
(thesaurus)
'regulation covering one police department making a comeback' is the wordplay.
'regulation' becomes 'rule' ('rule' can be a synonym of 'regulation'**).
'covering' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'police department' becomes 'cid' (I've seen this in other clues).
'making a comeback' is a reversal indicator.
'cid' in reverse letter order is 'dic'.
'i'+'dic'='idic'
'rule' placed around 'idic' is 'RIDICULE'.
(Other definitions for ridicule that I've seen before include "Meek" , "satire" , "guy" , "Lampoon" , "Mock, make fun of." .)