Japan's leader dubious, losing yen for a moment (4)
I believe the answer is:
jiff
'moment' is the definition.
(thesaurus)
'japan's leader dubious losing yen' is the wordplay.
'japan's leader' becomes 'j' (I can't explain this - if you can you should believe this answer much more).
'dubious' becomes 'iffy' (I've seen this in another clue).
'losing' is a deletion indicator.
'yen' becomes 'y' (resembles the symbol for the currency of Japan).
'iffy' with 'y' removed is 'iff'.
'j'+'iff'='JIFF'
'for a' acts as a link.
Can you help me to learn more?
(Other definitions for jiff that I've seen before include "Instant (sl)" , "A mo" , "Two shakes" , "Second" .)