Story with a lesson about a Tom hiding his head is comprehensible (10)
I believe the answer is:
fathomable
'comprehensible' is the definition.
(associated in meaning)
'story with a lesson about a tom hiding his head' is the wordplay.
'story with a lesson' becomes 'fable'.
'about' is an insertion indicator.
'hiding' means one lot of letters goes inside another.
'head' suggests taking the first letters.
The first letter of 'his' is 'h'.
'a'+'tom'='atom'
'fable' placed around 'atom' is 'fatomable'.
'fatomable' going around 'h' is 'FATHOMABLE'.
'is' acts as a link.
(Other definitions for fathomable that I've seen before include "Comprehensible" , "understandable" .)