Groom starts to rile everyone in prison (5)
I believe the answer is:
preen
'groom' is the definition.
(a bird grooms itself by preening feathers)
'starts to rile everyone in prison' is the wordplay.
'starts to' indicates taking the first letters.
'in' indicates putting letters inside.
'prison' becomes 'pen' (I've seen this before).
The initial letters of 'rile everyone' is 're'.
're' put into 'pen' is 'PREEN'.
(Other definitions for preen that I've seen before include "demonstrate pride" , "Primp" , "Groom with great care" , "setdown straight?" , "Perfect one's appearance" .)