For this penetrating word, I trust to love! (4)
I believe the answer is:
into
'for this penetrating word i trust' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'to love' is the wordplay.
'to' becomes 'int' (I am not sure about this - if you are sure you should believe this answer much more).
'love' becomes 'o' (In tennis, 'love' means 'zero').
'int'+'o'='INTO'
Can you help me to learn more?
(Other definitions for into that I've seen before include "To a point within - not I" , "Showing enthusiasm for" , "Liking" , "digging" , "ignition regularly missing" .)