Junior’s grabbing sword, descending on plants — yikes! (7,8)
I believe the answer is:
jeepers creepers
'yikes' is the definition.
(both are expressions of fear)
'junior's grabbing sword descending on plants' is the wordplay.
'junior's' becomes 'jr.'s' (abbreviation).
'grabbing' is an insertion indicator.
'sword' becomes 'eepe' (I can't explain this - if you can you should give a lot more credence to this answer).
'descending on' says to put letters next to each other (in a down clue, some letters go on top of others).
'plants' becomes 'creepers' (creeper is a type of plant).
'jrs' going around 'eepe' is 'jeepers'.
'jeepers'+'creepers'='JEEPERS CREEPERS'
(Another definition for jeepers creepers that I've seen is " Golly gosh!".)