Captain gets first class service making rock pile (5)
I believe the answer is:
cairn
'rock pile' is the definition.
'cairn' can be an answer for 'pile' (I have seen 'Pile of stones ' mean 'cairn' so perhaps 'pile' could also mean 'cairn'). I'm not sure about the 'rock' bit.
'captain gets first class service' is the wordplay.
'captain' becomes 'c' (this could be a standard abbreviation of which I'm unaware).
'gets' means one lot of letters go next to another.
'first class' becomes 'ai' (Ac - top quality).
'service' becomes 'RN' (abbreviation for Royal Navy).
'c'+'ai'+'rn'='CAIRN'
'making' acts as a link.
Can you help me to learn more?
(Other definitions for cairn that I've seen before include "Mound of stones on mountain top" , "Stone-pile monument" , "Stone-pile landmark" , "Man" , "Stones piled up as a marker" .)