Lend SMS a hidden meaning (7)
I believe the answer is:
subtext
'a hidden meaning' is the definition.
(secondary meaning behind something)
'lend sms' is the wordplay.
'lend' becomes 'sub' (to sub someone is to lend them money).
'sms' becomes 'text' (abbreviation for short message service).
'sub'+'text'='SUBTEXT'
(Other definitions for subtext that I've seen before include "Unwritten meaning" , "Something implied" , "Level of deeper meaning" , "Underlying theme" , "Implicit meaning" .)