Spike mark over one’s stomach (5)
I believe the answer is:
piton
'spike' is the definition.
(piton is a kind of spike)
'mark over one's stomach' is the wordplay.
'mark' becomes 'pit' (synonyms).
'over one's stomach' becomes 'on' (I can't justify this - if you can you should give a lot more credence to this answer).
'pit'+'on'='PITON'
Can you help me to learn more?
(Other definitions for piton that I've seen before include "Metal spike used in rock climbing" , "Peg driven into rock to aid climbing" , "something used when climbing" , "Climber's spike" , "something upwardly mobile type hammers home" .)