Tommy periodically retypes periodical (7,3)
I believe the answer is:
private eye
'periodical' is the definition.
(I've seen this before)
'tommy periodically retypes' is the wordplay.
'tommy' becomes 'private' (informal term for a British soldier).
'periodically' indicates alternate letters (letters must be selected 'every so often').
The alternate letters of 'retypes' are 'eye'.
'private'+'eye'='PRIVATE EYE'
(Other definitions for private eye that I've seen before include "Periodical" , "supplement to work of police?" , "Investigative satirical magazine" , "investigator" , "Satirical magazine - undercover detective" .)