Stick knife into second fruit (5)
I believe the answer is:
spear
'stick knife' is the definition.
'spear' can be an answer for 'knife' (thesaurus). I am not certain of the 'stick' bit.
'second fruit' is the wordplay.
'second' becomes 's' (common abbreviation).
'fruit' becomes 'pear' (pear is a kind of fruit).
's'+'pear'='SPEAR'
'into' is the link.
Can you help me to learn more?
(Other definitions for spear that I've seen before include "Sharp weapon" , "Thrown weapon" , "Portion of asparagus" , "stick" , "Asparagus shoot" .)