Gym equipment? Cargo boat has old stock on hand (10)
I believe the answer is:
trampoline
'gym equipment?' is the definition.
'trampoline' can be an answer for 'equipment?' (trampoline is a kind of equipment). I'm not certain of the 'gym' bit.
'cargo boat has old stock on hand' is the wordplay.
'cargo boat' becomes 'tramp' (I've seen this before).
'has' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'stock on hand' becomes 'line' (I can't explain this - if you can you should give a lot more credence to this answer).
'tramp'+'o'+'line'='TRAMPOLINE'
Can you help me to learn more?
(Other definitions for trampoline that I've seen before include "Sprung gymnastic equipment" , "Spring up" , "Trap oilmen (anag.)" , "bouncer" , "On which bouncers go" .)