Tiny boy with cat fathoms out chronological list (8)
I believe the answer is:
timeline
'chronological list' is the definition.
(a list of events in chronological order)
'tiny boy with cat fathoms out' is the wordplay.
I cannot quite see how this works, but
'boy' could be 'mel' and 'mel' is found within the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for timeline that I've seen before include "Representation of events in order" , "Chronology" , "Events laid out linearly" .)