Entertainer set off after lorry, cape having fallen off the back (7)
I believe the answer is:
artiste
'entertainer' is the definition.
(artiste is a kind of entertainer)
'set off after lorry cape having fallen off the back' is the wordplay.
'off' indicates anagramming the letters.
'after' means one lot of letters go next to another.
'lorry' becomes 'artic' (articulated lorry is a semi-trailer truck).
'cape' becomes 'c' (abbreviation used on maps).
'having fallen off the back' indicates named letters should be taken away.
'artic' with 'c' removed is 'arti'.
'set' anagrammed gives 'ste'.
'ste' after 'arti' is 'ARTISTE'.
(Other definitions for artiste that I've seen before include "Person playing" , "One entertaining" , "Professional performer" , "A sitter for creative person" , "Player" .)