Feeble comedian with base lines (7)
I believe the answer is:
doddery
'feeble' is the definition.
(thesaurus)
'comedian with base lines' is the wordplay.
'comedian' becomes 'Dodd' (Ken Dodd).
'with' says to put letters next to each other.
'base' becomes 'e' (Euler's number - base of natural logarithms).
'lines' becomes 'ry' (abbreviation for railway).
'dodd'+'e'+'ry'='DODDERY'
(Other definitions for doddery that I've seen before include "Shaky" , "Old and feeble" , "Rather senile" , "Infirm with age" , "Old and infirm" .)