Athletic little chap crossing sierra: seamstresses may use it (7,6)
I believe the answer is:
running stitch
'seamstresses may use it' is the definition.
I can't tell whether this definition defines the answer.
'athletic little chap crossing sierra' is the wordplay.
'athletic' becomes 'running' (I can't explain this - if you can you should give a lot more credence to this answer).
'little chap' becomes 'titch' (term for a short person).
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'sierra' becomes 'S' (phonetic alphabet: alpha, bravo, charlie etc.).
'running'+'titch'='runningtitch'
'runningtitch' going around 's' is 'RUNNING STITCH'.
Can you help me to learn more?
(Other definitions for running stitch that I've seen before include "Basic needlework style" , "continuous pain in side" , "Joiner" .)