Maybe old farm worker’s beginning to regret cutting into pipe (5)
I believe the answer is:
horse
'maybe old farm worker's beginning' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'regret cutting into pipe' is the wordplay.
'regret' becomes 'r' (this could be a standard abbreviation I've not previously seen).
'cutting into' means one lot of letters goes inside another.
'pipe' becomes 'hose' (I've seen this before).
'r' inserted within 'hose' is 'HORSE'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for horse that I've seen before include "Stallion, e.g." , "Animal featuring in the sport of kings" , "Hoofed mammal" , "it can become addictive" , "drug?" .)