Support the old man — it's right to get stuck in (4)
I believe the answer is:
prop
'support' is the definition.
(prop up can mean to support)
'old man it's right to get stuck in' is the wordplay.
'old man' becomes 'pop' (synonyms).
'it's right' becomes 'r' (common abbreviation).
'to get stuck in' is an insertion indicator.
'pop' going around 'r' is 'PROP'.
'the' acts as a link.
(Other definitions for prop that I've seen before include "needed at certain stages" , "Support; rugby forward" , "Support to keep from falling" , "Seize and hold firmly" , "A rigid support - 37 Across is one" .)