A particular chap, fifty-nine in spring (5)
I believe the answer is:
helix
'spring' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'a particular chap fifty-nine' is the wordplay.
'a particular chap' becomes 'he' (I am not sure about the 'particular' bit.**).
'fifty' becomes 'l' ('L' can be a synonym of 'fifty').
'nine' becomes 'ix' ('ix' can be a synonym of 'nine').
'he'+'l'+'ix'='HELIX'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for helix that I've seen before include "'Spiral structure, DNA is a double one (5)'" , "Screwed coil" , "that goes round and round and up and down" , "Structure consisting of something wound in a continuous series of loops" , "A three-dimensional spiral" .)