Image principally found in tramp's file (7)
I believe the answer is:
dossier
'file' is the definition.
(thesaurus)
'image principally found in tramp's' is the wordplay.
'image principally' becomes 'i' (I am not sure about this - if you are sure you should believe this answer much more).
'found in' indicates putting letters inside.
'tramp' becomes 'dosser' (dosser is a kind of tramp).
'i' inserted into 'dosser' is 'DOSSIER'.
Can you help me to learn more?
(Other definitions for dossier that I've seen before include "Report" , "File of documents" , "Papers containing detailed information about a particular person" , "Set of documents about person" , "A collection of papers" .)