TV pioneer initially active in prison (5)
I believe the answer is:
baird
'tv pioneer' is the definition.
(I've seen this in another clue)
'initially active in prison' is the wordplay.
'initially' says to take the initial letters.
'in' indicates putting letters inside.
'prison' becomes 'bird'.
The first letter of 'active' is 'a'.
'a' inserted inside 'bird' is 'BAIRD'.
(Other definitions for baird that I've seen before include "Scottish inventor" , "He devised television" , "Television pioneer" , "inventive man" , "John Logie - -" .)